Lesson 1: Fundamentals of Wheeled Vehicle Braking Systems
Braking action on wheeled vehicles is the use of a controlled force to hold, stop, or reduce the speed of a vehicle. Many factors must be considered when designing the braking system for an automotive item. The vehicle weight, size of tires, and type of suspension are but a few that influence the design of a system.
The power needed to brake a vehicle is equal to that needed to make it go. However, for safety reasons, brakes must be able to stop the car in a very short distance. As an example, a passenger car equipped with an 80-HP engine can normally accelerate from a standstill to 60 MPH in about 36 seconds. On the other hand, the brakes must be able to decelerate the vehicle from 60 MPH to a stop in 4 1/2 seconds. You can therefore see the braking force is about eight times greater than the power developed by the engine.
Each part in the braking system must operate with a very positive action to accomplish this tremendous braking effort. The job of a wheeled vehicle mechanic is to maintain the braking components in a state of repair that ensures serviceable brakes when needed. For you to keep brake system components in a working shape, you must understand how the system works. In this lesson, we will discuss the principles of operation for components contained in various types of braking systems.
Braking action is the use of a controlled force to slow the speed of or stop a moving object, in this case a vehicle. It is necessary to know what friction is to understand braking action.
Friction is the resistance to movement between two surfaces or objects that are touching each other. An example of friction is the force which tries to stop your hand as you apply pressure and slide it across a table or desk. This means that by forcing the surface of an object that is not moving (stationary) against a moving object's surface, the resistance to movement or the rubbing action between the two surfaces of the objects will slow down the moving surface. Automotive vehicles are braked in this manner.
PRINCIPLES OF BRAKING
FIGURE 1. DEVELOPMENT OF FRICTION AND HEAT.
Brakes on early motor vehicles were nothing more than modified wagon brakes used on horse-drawn wagons. These were a hand-operated, mechanical, lever-type brakes that forced a piece of wood against one or more of the wheels. This caused friction or a drag on the wheel or wheels.
There is also friction between the wheel and ground that tries to prevent the wheel from sliding or skidding on the ground. When a vehicle is moving, there is a third force present. This force is known as kinetic energy. This is the name given the force that tries to keep any object in motion once it has started moving.
When the brakes are applied, the wheel will either roll or skid, depending on which is greater, the friction between the braking surfaces or between the wheel and the road. Maximum retardation (slowing down) is reached when friction between the brake surfaces is just enough to almost lock the wheel. At this time, friction between the brake surfaces and wheel and road are almost the same. This is all the friction that can be used in retarding (slowing down) the motion of the vehicle. The amount of friction between the road and the wheel is what limits braking. Should friction between the braking surfaces go beyond this, the braking surfaces will lock and the wheels will skid.
When a wheel rolls along a road, there is no movement between (relative motion) the wheel and road at the point where the wheel touches the road. This is because the wheel rolls on the road surface; but, when a wheel skids, it slides over the surface of the road, and there is relative motion because the wheel is not turning while moving over the road. When a wheel skids, friction is reduced, which decreases the braking effect. However, brakes are made so that the vehicle operator is able to lock the wheels if enough force to the brake lever or pedal is applied.
FIGURE 2. BRAKING REQUIREMENTS.
Most of us know that to increase a vehicle's speed requires an increase in the power output of the engine. It is just as true that an increase in speed requires an increase in the braking action necessary to bring a vehicle to a stop. Brakes must not only be able to stop a vehicle, but must stop it in as short a distance as possible. Because brakes are expected to decelerate (slow down) a vehicle at a faster rate than the engine can accelerate it, they must be able to control a greater power than that developed by the engine. This is the reason that well-designed, powerful brakes have to be used to control the modern high-speed motor vehicle. The time needed to stop is one-eighth the time needed to accelerate from a standing start. The brakes then can handle eight times the power developed by the engine.
FACTORS CONTROLLING RETARDATION
The amount of retardation (slowing down) obtained by the braking system of a vehicle is affected by several factors. For wheel brakes used on today's motor vehicles, these factors are the pressure exerted on the braking surfaces (lining and drum), the weight carried on the wheel, the overall radius of the wheel (the distance from the center of the wheel to the outer tread of the tire), the radius of the brake drum, the amount of friction between the braking surfaces, and the amount of friction between the tire and the road. The amount of friction between the tire and the road determines the amount of retardation that can be obtained by the application of the brakes. The things that affect the amount of friction between the tires and the road are the amount and type of tread in contact with the road surface and the type and condition of the road surface. There will be much less friction, and thus much less retardation, on wet or icy roads than on good dry roads.
DRIVER'S REACTION TIME
FIGURE 3. TOTAL VEHICLE STOPPING DISTANCE OF AN AVERAGE VEHICLE.
Another factor that affects the time and distance required to bring a vehicle to a stop is the driver's reaction time. Reaction time is the time required for the driver to move his/her foot from the accelerator pedal to the brake pedal and apply the brakes. While the driver is thinking of applying the brakes and reacting to do so, the vehicle will move a certain distance. How far it will move depends on its speed. After the brakes are applied, the vehicle will travel an additional distance before it is brought to a stop. The total stopping distance of a vehicle is the total of the distance covered during the driver's reaction time and the distance during which the brakes are applied before the vehicle stops. This illustration shows the total stopping distance required at various vehicle speeds. This is assuming an average reaction time of three-quarters of a second and that good brakes are applied under the most favorable road conditions.
EXTERNAL-CONTRACTING AND INTERNAL-EXPANDING BRAKES
FIGURE 4. EXTERNAL-CONTRACTING AND INTERNAL-EXPANDING BRAKES.
There are several types of braking systems. All systems require the use of a rotating (turning) unit and a nonrotating unit. Each of these units contains braking surfaces that, when rubbed together, give the braking action. The rotating unit on military wheeled vehicle brakes consists of a drum secured to the wheel. The nonrotating unit consists of brake shoes and the linkage needed to apply the shoes to the drum. Brakes are either the external-contracting or internal-expanding type, depending on how the nonrotating braking surface is forced against the rotating braking surface.
When a brake shoe or a brake band is applied against the outside of a rotating brake drum, the brake is known as an external-contracting brake. On this type of brake, the nonrotating braking surface must be forced inward against the drum to produce the friction necessary for braking. The brake band is tightened around the drum by moving the brake lever. Unless an elaborate cover is provided, the external-contracting brake is exposed to dirt, water, and other foreign matter which rapidly wears the lining and drum. This is particularly true with wheel brakes.
The nonrotating unit may be placed inside the rotating drum with the drum acting as a cover for the braking surfaces. This type of brake is known as an internal-expanding brake because the nonrotating braking surface is forced outward against the drum to produce braking action. This type of brake is used on the wheel brakes of cars and trucks because it permits a more compact and economical construction. The brake shoes and brake-operating mechanism may be mounted on a backing plate or brake shield made to fit against and close the open end of the brake drum. This protects the braking surfaces from dust and other foreign matter.
Some vehicles are fitted with a third type of brake system known as disk brakes. The rotating member is known as the rotor. A brake pad is positioned on each side of the rotor. The brakes operate by squeezing together and grasping the rotor to slow or stop the disk.
FIGURE 5. BRAKE DRUM CONSTRUCTION.
The brake drums are usually made of pressed steel, cast iron, or a combination of the two metals. Cast-iron drums dissipate the heat produced by friction more rapidly than steel drums and have better friction surfaces. However, if a cast-iron drum is made as strong as it should be, it will be much heavier than a steel drum.
To provide light weight and enough strength, some drums are made of steel with a cast-iron liner for the braking surface. This type is known as a centrifuse brake drum. Cooling ribs are sometimes added to the outside of the drum to give more strength and better heat dissipation. Braking surfaces of drums may be ground, or they may be machined to a smooth finish.
For good braking action, the drum should be perfectly round and have a uniform surface. Brake drums become "out of round" from pressure exerted by the brake shoes or bands and from the heat produced by the application of the brakes. The brake drum surface becomes scored when it is worn by the braking action. When the surface is badly scored or the drum is out of round, it is necessary to replace the drum or regrind it or turn it down in a lathe until the drum is again smooth and true.
FIGURE 6. BRAKE SHOES AND BRAKE LININGS.
Brake shoes are made of malleable iron, cast steel, drop-forged steel, pressed steel, or cast aluminum. Pressed steel is usually used because it is cheaper to produce in large quantities. Steel shoes expand at approximately the same rate as the drum when heat is produced by brake application, thereby maintaining the clearance between the brake drum and the brake shoe under most conditions.
A friction lining riveted or bonded to the face of the shoe makes contact with the inner surface of the brake drum when the brake is applied. On the riveted-type lining, brass rivets are usually used because brass does not unduly score the drum when the lining is worn. Aluminum rivets are not very satisfactory because they are corroded very readily by salt water. The bonded lining is not riveted but is bonded directly to the shoe with a special cement.
Differences in brake design and conditions of operation make it necessary to have various types of brake linings.
ROTATING AND NONROTATING UNITS
The brake drum is mounted directly onto the wheel and provides the rotating braking surface. The brake shield, sometimes known as the backing plate or dust shield, is mounted on some fixed structure such as the axle housing. The brake shield forms a support for the nonrotating braking surface (brake shoes) and its operating mechanism.
The brake shoes may be anchored to the brake shield by separate pins or the same pin. Springs or clips are usually used to hold the shoes close to the brake shield and to prevent them from rattling. A fairly strong retracting spring is hooked between the shoes to pull them away from the drum when the brakes are released. With a mechanical hookup, pressure can be applied to the brake shoes by means of a cam, toggle, or double-lever arrangement. A cam turned by a small lever is the method most frequently used. Turning the cam by the lever tends to spread the brake shoes and push them outward against the drum. With the hydraulic system, pressure is applied to the brake shoes by means of a cylinder and pistons.
FIGURE 7. SELF-ENERGIZING AND SERVO ACTION.
The brake operating linkage alone does not provide enough mechanical advantage for good braking. Some way of increasing the pressure of the brake shoes is needed. A self-energizing action can be used to do this, once the setting of the shoes is started by the movement of the linkage. There are several variations of this self-energizing action, but it is always done by the shoes themselves as they tend to turn with the turning drum.
When the brake shoe is anchored and the drum turns in the direction shown, the shoe will tend to turn with the drum when it is forced against the drum. Friction is trying to cause the shoe to turn with the drum. When this happens, the shoe pushes against the anchor pin. Since the pin is fixed to the brake shield, this pressure tends to wedge the shoe between the pin and drum. As the cam increases the pressure on the shoes, the wedging action increases and the shoe is forced still more tightly against the drum to increase the friction. This self-energizing action results in more braking action than could be obtained by the pressure of the cam against the shoes alone. Brakes making use of this principle to increase pressure on the braking surfaces are known as self-energizing brakes.
It is very important that the operator control the total braking action at all times, which means the self-energizing action should increase only upon the application of more pressure on the brake pedal. The amount of self-energizing action available depends mainly on the location of the anchor pin. As the pin is moved toward the center of the drum, the wedging action increases until a point is reached where the shoe will automatically lock. The pin must be located outside this point so that the operator can control the braking.
When two shoes are anchored on the bottom of the brake shield, self-energizing action is effective on only one shoe. The other shoe tends to turn away from its pivot. This reduces its braking action. When the wheel is turning in the opposite direction, the self-energizing action is produced on the opposite shoe.
Two shoes can be mounted so that self-energizing action is effective on both. This is done by pivoting the shoes to each other and leaving the pivot free of the brake shield. The only physical effort required is for operating the first or primary shoe. Both shoes then apply more pressure to the braking surfaces without an increase in pressure on the brake pedal. The anchor pins are fitted into slots in the free ends of the brake shoes. This method of anchoring allows the shoes to move and expand against the drum when the brakes are applied. The self-energizing action of the primary shoe is transmitted through the pivot to the secondary shoe. Both shoes will tend to turn with the drum and will be wedged against the drum by one anchor pin. The other anchor pin will cause a similar action when the wheel is turning in the opposite direction.
Another type of brake shoe that has been used consists of two links anchored together on the brake shield with the end of each link pivoted to one of the brake shoes. This allows more even application of the braking surface because of the freedom of movement for the brake shoes. Each shoe is self-energizing in opposite directions.
FIGURE 8. DISK BRAKE ASSEMBLY.
The disk brake, like the drum brake assembly, is operated by pressurized hydraulic fluid. The fluid, which is routed to the calipers through steel lines and flexible high-pressure hoses, develops its pressure in the master cylinder. Once the brake pedal is depressed, fluid enters the caliper and begins to force the piston(s) outward. This outward movement forces the brake pads against the moving rotor. Once this point is reached, the braking action begins. The greater the fluid pressure exerted on the piston(s) from the master cylinder, the tighter the brake pads will be forced against the rotor. This increase in pressure also will cause an increase in braking effect. As the pedal is released, pressure diminishes and the force on the brake pads is reduced. This allows the rotor to turn more easily. Some calipers allow the brake pads to rub lightly against the rotor at all times in the released position. Another design uses the rolling action of the piston seal to maintain a clearance of approximately 0.005 inches when the brakes are released.
Comparison to Drum Brakes
Both the disk and brake drum assemblies used on modern vehicles are well-designed systems. Each system exhibits certain inherent advantages and disadvantages. The most important points of interest are discussed below. One major factor that must be discussed in automotive brakes, as well as all other brake systems, is the system's ability to dissipate heat. As discussed previously, the byproduct of friction is heat. Because most brake systems use this concept to develop braking force, it is highly desirable for brake systems to dissipate heat as rapidly and efficiently as possible. The disk brake assembly, because of its open design, has the ability to dissipate heat faster than the brake drum. This feature makes the disk brake assembly less prone to brake fade due to a buildup of excess heat. The disk assembly also may have additional heat transfer qualities due to the use of a ventilated rotor. This type of rotor has built-in air passages between friction surfaces to aid in cooling.
While the brake drum assembly requires an initial shoe-to-drum clearance adjustment and periodic checks, the disk brake assembly is self-adjusting and maintains proper adjustment at all times. The disk assembly automatically compensates for lining wear by allowing the piston in the caliper to move outward, thereby taking up excess clearance between pads and rotor.
The disk system is fairly simplistic in comparison to the drum system. Due to this design and its lack of moving parts and springs, the disk assembly is less likely to malfunction. Over-hauling the disk brake assembly is faster because of its simplistic design. It also is safer due to the fact that the disk brake assembly is open and asbestos dust from linings is less apt to be caught in the brake assembly. Like brake drums, rotors may be machined if excessive scoring is present. Rotors also are stamped with a minimum thickness dimension which should not be exceeded. The drum brake assembly requires that the drum be removed for lining inspection, while some disk pads have a built-in lining wear indicator that produces an audible high-pitch squeal when linings are worn excessively. This harsh squeal is a result of the linings wearing to a point, allowing a metal indicator to rub against the rotor as the wheel turns. Because of its small frictional area and lack of self-energizing and servo effect, the disk brake assembly requires the use of an auxiliary power booster to develop enough hydraulic pressure for satisfactory braking.
The floating caliper is designed to move laterally on its mount. This movement allows the caliper to maintain a centered position with respect to the rotor. This design also permits the braking force to be applied equally to both sides of the rotor. The floating caliper usually is a one-piece solid construction and uses a single piston to develop the braking force. This type of caliper operates by pressurized hydraulic fluid like all other hydraulic calipers. The fluid under pressure first enters the piston cavity and begins to force the piston outward. As this happens, the brake pad meets the rotor. Additional pressure then forces the caliper assembly to move in the opposite direction of the piston, thereby forcing the brake pad on the opposite side of the piston to engage the rotor. As pressure is built up behind the piston, it then forces the brake pads tighter against the rotor to develop additional braking force.
The fixed caliper is mounted rigidly to the spindle or splash shield. In this design, the caliper usually is made in two pieces and has either two, three, or four pistons in use. The pistons, which may be made of cast iron, aluminum, or plastic, are provided with seals and dust boots and fit snugly in bores machined in the caliper. The centering action of the fixed caliper is accomplished by the pistons as they move in their bores. If the lining should wear unevenly on one side of the caliper, the excess clearance would be taken up by the piston simply by moving further out in its bore. As the brakes are applied, the fluid pressure enters the caliper on one side and is routed to the other through an internal passageway or an external tube connected to the opposite half of the caliper. As pressure is increased, the pistons force the brake pads against the rotors evenly and therefore maintain an equal amount of pressure on both sides of the rotor.
As discussed above, the fixed calipers use a multi-piston design to provide the braking force. The fixed calipers may be designed to use two, three, or four pistons. The dual-piston design provides a slight margin of safety over a single-piston floating caliper. In the event of a piston seizing in the caliper, the single-piston caliper would be rendered useless, while the dual-piston design would still have one working piston to restore some braking ability. The three- and four-piston design provides for the use of a large brake lining. The brake force developed may now be spread over a larger area of the brake pad.
MECHANICAL BRAKE SYSTEMS
On wheeled vehicles, the energy supplied by the operator's foot pushing down on the brake pedal is transferred to the brake mechanism on the wheels by various means. A mechanical hookup was used on the first motor vehicles. Now, mechanically-operated braking systems are practically obsolete. However, mechanical hookups are still used for a part of the braking systems in many vehicles.
The parking brake (auxiliary brake) is generally used to lock the rear wheels or propeller shafts of a vehicle to prevent the vehicle from rolling when it is parked. It can also be used to stop the vehicle in an emergency if the service brakes fail. For this reason, the parking brake is sometimes referred to as the emergency brake.
FIGURE 9. EXTERNAL-CONTRACTING BRAKE.
The M151-series trucks use an external-contracting-type brake that is mounted on the transmission or transfer. It has a brake drum that is splined and bolted to a transfer output shaft. A flexible brake band with internal lining is located around the outer circle of the drum. The brake is made to be self-energizing by either forward or backward movement of the vehicle. For this reason, the brake band is anchored at a point just opposite from the point where the operating force is applied. One-half of the band will then wrap tighter (self-energize) on the drum in one direction and the other half in the opposite direction. The mechanism for operating the brake is usually a simple bell crank arrangement controlled by a hand lever. Applying the brake locks the transmission or transfer output shaft, which, in turn, locks the propeller shaft holding the wheels through the axle assembly. When the hand lever is in the released position, the brake band is released by spring pressure.
The parking brake system of the M880- and M1008-series vehicles uses the rear wheel drum brakes to hold the vehicle motionless. When the operator of the vehicle applies the parking brake, the effort with which the brake lever is moved is transmitted to the rear shoes by cables. Levers in the system multiply the physical effort of the operator enough to force the rear brake shoes into tight contact with the drums.
The parking brake system of the M998-series vehicles use a disk mounted on the rear differential propeller shaft to hold the vehicle motionless. When the operator of the vehicle applies the parking brake, a mechanical linkage multiplies the force of the operator, and transmits this increased pressure to the brake unit. The brake unit uses the force to push the brake pads against the drum.
Some large trucks use a parking brake that has a drum with internal-expanding brake shoes similar to the service brakes. Braking action is obtained by clamping the rotating drum between two brake shoes. The lining on the brake shoes contacts the friction surfaces of the drum.
The 2 1/2- and 5-ton military trucks have a parking brake that operates by clamping the flange of a drum between brake shoes. Although it is constructed somewhat different, it uses the same operating principles as the disk brake. The brake is mounted on the rear of the transfer and locks the wheels through the axle assemblies and propeller shafts. The drum has a flange with both inner and outer braking surfaces. Brake shoes with linings are located on the inside as well as the outside of the drum. The outer brake shoe is supported by the pivots on an anchor at its lower end. The inner brake shoe is supported by the brake shoe lever, which is pinned to the center of both the outer and inner brake shoes. Pulling the brake shoe lever moves the brake shoes together clamping the drum flange between them.
FIGURE 10. HYDRAULIC BRAKE SYSTEM.
In hydraulic braking systems, the pressure applied at the brake pedal is transmitted to the brake mechanism by a liquid. Since a liquid cannot be compressed under ordinary pressures, force is transmitted solidly just as if rods were used. Force exerted at any point upon a confined liquid is distributed equally through the liquid in all directions so that all brakes are applied equally.
In a hydraulic brake system, the force is applied to a piston in a master cylinder. The brake pedal operates the piston by linkage. Each wheel brake is provided with a cylinder. Inside the cylinder are opposed pistons which are connected to the brake shoes. When the brake pedal is pushed down, linkage moves the piston within the master cylinder, forcing the brake liquid or fluid from the cylinder. From the master cylinder, the fluid travels through tubing and flexible hose into the four wheel cylinders.
The brake fluid enters the wheel cylinders between the opposed pistons. The pressure of the brake fluid on the pistons causes them to move out. This forces the brake shoes outward against the brake drum. As pressure on the pedal is increased, more hydraulic pressure is built up in the wheel cylinders and more force is exerted against the ends of the brake shoes.
When the pressure on the pedal is released, retracting (return) springs on the brake shoes pull the shoes away from the drum. This forces the wheel cylinder pistons to their release positions and also forces the brake fluid back through the flexible hose and tubing to the master cylinder.
FIGURE 11. MASTER CYLINDER.
The master cylinder housing is an iron casting which contains the cylinder and a large reservoir for the brake fluid. The reservoir carries enough reserve fluid to ensure proper operation of the braking system. It is filled through a hole at the top which is sealed by a removable filler cap containing a vent. The cylinder is connected to the reservoir by two drilled holes or ports, a large intake port, and a small bypass port.
The master cylinder piston is a long, spool-like member with a rubber secondary cup seal at the outer end and a rubber primary cup which acts against the brake liquid just ahead of the inner end. The primary cup is kept against the end of the piston by a return spring. The inner piston head has several small bleeder ports that pass through the head to the base of the rubber primary cup. A steel stop disk, held in the outer end of the cylinder by a retaining spring (snap ring), acts as a piston stop. A rubber boot covers the piston end of the master cylinder to prevent dust and other foreign matter from entering the cylinder. This boot is vented to prevent air from being compressed within it.
In the outlet end of the cylinder is a combination inlet and outlet valve which is held in place by the piston return spring. This check valve is a little different from most check valves that will let fluid pass through them in one direction only. If enough pressure is applied to this valve, fluid can go either through or around it in either direction. This means it will keep some pressure in the brake lines. The check valve consists of a rubber valve cup inside a steel valve case which seats on a rubber valve seat that fits in the end of the cylinder. In some designs, the check valve consists of a spring-operated outlet valve seated on a valve cage rather than a rubber cup outlet valve. The principle of operation is the same. The piston return spring normally holds the valve cage against the rubber valve seat to seal the brake fluid in the brake line.
FIGURE 12. WHEEL CYLINDER.
The wheel cylinder changes hydraulic pressure into mechanical force that pushes the brake shoes against the drum. The wheel cylinder housing is mounted on the brake backing plate. Inside the cylinder are two pistons which are moved in opposite directions by hydraulic pressure and which, at the same time, push the shoes against the drum. The piston or piston stems are connected directly to the shoes. Rubber piston cups fit in the cylinder bore against each piston to prevent the escape of brake liquid. There is a light spring between the cups to keep them in position against the pistons. The open ends of the cylinder are fitted with rubber boots to keep out foreign matter. Brake fluid enters the cylinder from the brake line connection between the pistons. At the top of the cylinder, between the pistons, is a bleeder hole and screw through which air is released when the system is being filled with brake fluid.
On some vehicles, a stepped wheel cylinder is used to compensate for the faster rate of wear on the front shoe than on the rear shoe. This happens because of the self-energizing action. By using a larger piston for the rear shoe, the shoe receives more pressure to offset the self-energizing action of the front shoe.
If it is desired that both shoes be independently self-energizing, it is necessary to have two wheel cylinders, one for each shoe. Each cylinder has a single piston and is mounted on the opposite side of the brake backing plate from the other cylinder.
So far, we have discussed the parts needed to make up a hydraulic brake system. Now let's see what happens to these parts when the brakes are applied and released. Let's assume the master cylinder is installed on a vehicle and the hydraulic system is filled with fluid. As the driver pushes down on the brake pedal, linkage moves the piston in the master cylinder. As the piston moves inward, the primary cup seals off the bypass port (sometimes known as the compensating port).
With the bypass port closed, the piston traps the fluid ahead of it and creates pressure in the cylinder. This pressure forces the check valve to open and fluid passes into the brake line. As the piston continues to move, it forces fluid through the lines into the wheel cylinders. The hydraulic pressure causes the wheel cylinder pistons to move outward and force the brake shoes against the brake drum. As long as pressure is kept on the brake pedal, the shoes will remain pressed against the drum.
When the brake pedal is released, the pressure of the link or pushrod is removed from the master cylinder piston. The return spring pushes the piston back to the released position, reducing the pressure in front of the piston. The check valve slows down the sudden return of fluid from the wheel cylinders. As the piston moves toward the released position in the cylinder, fluid from the master cylinder supply tank flows through the intake port and then through the bleeder holes in the head of the piston. This fluid will bend the lips of the primary cup away from the cylinder wall, and the fluid will flow into the cylinder ahead of the piston.
When the pressure drops in the master cylinder, the brake shoe return springs pull the shoes away from the drum. As the shoes are pulled away from the drum, they squeeze the wheel cylinder pistons together. This forces the brake fluid to flow back into the master cylinder.
The returning fluid forces the check valve to close. The entire check valve is then forced off its seat, and fluid flows into the master cylinder around the outer edges of the valve. When the piston in the master cylinder has returned to its released position against the stop plate, the primary cup uncovers the bypass port and any excess fluid will flow through the bypass port to the reservoir. This prevents the brakes from "locking up" when the heat of the brakes causes the brake fluid to expand.
When the piston return spring pressure is again more than the pressure of the returning fluid, the check valve seats. The valve will keep a slight pressure in the brake lines and wheel cylinders. The brake system is now in position for the next brake application.
The hydraulic braking system of the modern high-speed automobile must be kept in a high state of repair. Not too many years ago, a small defect in the braking system did not bother too much; in fact, it might not even have been noticed. Today, however, improved road conditions and higher vehicle speeds, plus more sensitive steering and suspension systems, cause a poor braking action to be noticed immediately. The brake system parts must be able to stand up under high pressures and temperatures and still be able to work properly if the vehicle is to be operated safely. To properly repair a brake system so it is always in top condition, the mechanic must be well trained and have a desire to do the best job possible.
BEFORE ROAD-TEST INSPECTION
The condition of the hydraulic service brakes of a vehicle can be determined by inspecting the following items: fluid level in the master cylinder, brake pedal free travel, total brake pedal travel, feel of brake pedal (hard or spongy), leaks in the hydraulic system, noise during operations, performance, and the amount of wear of brake parts. Wear can normally be determined by checking one wheel of each axle.
To inspect the fluid level in the master cylinder, first clean away all dirt that may fall into the master cylinder reservoir. Remove the filler cap and ensure the fluid level is at the level recommended in the maintenance manual pertaining to the vehicle being serviced. The level of fluid is determined by measuring the distance from the top of the filler hole to the level of fluid in the reservoir.
If the fluid level is low, refer to the vehicle's lubrication order for the recommended type of brake fluid and add fluid as needed. Since the end of 1982, all military vehicles use silicone brake fluid. Silicone fluid does not absorb water, provides good corrosion protection, and has good lubrication qualities. The fluid is also compatible with the rubber components of the brake system.
Check the master cylinder supply tank reservoir vent to make sure that it is not plugged. On some vehicles, a small hole drilled in the filler cap vents the supply tank. On other vehicles, the supply tank is vented through a line and fitting connected to the top of the master cylinder supply tank. A plugged vent can be easily cleared with compressed air.
Measure the brake pedal free travel and compare the measurement with the specifications given in the vehicle's maintenance manual. Brake pedal free travel is the amount that the brake pedal can be moved without moving the master cylinder piston. If the pedal has too much free travel, it will have to be pushed farther before the brakes apply. If there is not enough free travel, it may prevent the brakes from releasing.
To check the total travel of the brake pedal, push the brake pedal down as far as you can. You should not be able to push the brake pedal on most trucks any closer to the floorboard than 2 inches.
- If there is too much pedal travel, but the pedal feels firm, the problem is probably caused by normal wear of the brake lining. When the lining is not worn too badly, an adjustment of the brake shoes will correct excessive pedal travel. Unfortunately, the only way to determine the exact amount of the brake lining wear on most vehicles is to remove the wheels and brake drums.
- If the pedal travel is too great and the pedal feels spongy, there is probably some air in the hydraulic system. Air trapped in the hydraulic system can be compressed and does not permit pressure applied to the pedal to be applied solidly to the brakes. Methods of correcting these problems are covered later in this lesson.
Inspect the hydraulic system for leaks. Large leaks can be detected while checking the pedal travel. This is done by holding a steady pressure on the brake pedal for a few moments. If the pedal continues to move down, there is a large leak. Small leaks cannot be detected this way as they cause the pedal to fall away too slowly to be noticed.
Look the entire hydraulic system over for any visible indications of leakage. Inspect the master cylinder, especially around the rubber boot, for external fluid leaks. Inspect all steel lines (tubes) for leakage, loose fittings, wear, dents, corrosion, and missing retaining clips. Inspect the flexible hoses for leakage, cuts, cracks, twists, and evidence of rubbing against other parts. Inspect the area at the lower edge of the backing plate for the presence of any brake fluid or grease. Leakage of either brake fluid or grease at the wheels is an indication of brake problems.
Road-test the vehicle and check the operation of the brakes by stopping several times while traveling on a smooth road. Check for squeaking or grinding sounds when the brakes are applied, an excessive amount of pressure required on the brake pedal to stop the vehicle, and the vehicle pulling to one side when the brakes are applied (uneven braking).
If the brakes make a squeaking or grinding sound, some of the more common causes are:
The wheel and brake drum will have to be removed from the noisy brake and the brake parts inspected to determine the exact cause of the noise.
When excessive pressure must be applied to the brake pedal to stop the vehicle, any one or more of the following items may be the cause:
AFTER ROAD-TEST INSPECTION
If such things as pulling to one side or poor braking action are noted during a road test, an after road-test inspection is done.
Many faults occur in the brake system that can cause the vehicle to pull to one side. However, all these faults have one thing in common: they affect the brake in one wheel causing that wheel to hold either more or less than the other wheels. If the affected wheel holds more, the vehicle will pull toward the affected wheel; if it holds less, the vehicle pulls away from the affected wheel. The most common faults that cause the brakes to hold unevenly are unequal brake adjustment, grease or brake fluid on the lining, dirt in the brake drum, brake drum or rotor scored or rough, different kinds of brake linings on opposite wheels, primary and secondary brake shoes reversed in one wheel (on some vehicles), glazed or worn lining, restricted brake line, weak brake shoe return springs, or sticking pistons in a wheel or caliper cylinder.
If the inspection indicates that the wheel brakes are at fault, you must determine the condition of the brake parts in the wheel brakes. Do this by removing one wheel and brake drum from each axle assembly and inspecting the brake parts in these wheels. It is reasonable to assume that the condition of both brake assemblies on one axle will be about the same. Inspect the condition of the brake drum, brake lining, brake shoe anchor, hold-down springs, retracting (return) springs, brake shoe adjusting mechanism, and wheel cylinder.
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