As a certified mechanic, you will be responsible for the maintenance, repair, and troubleshooting of braking systems. Braking systems are usually inspected yearly or every 12,000 miles to ensure safe operation, to comply with state and local regulations, and to keep personnel and equipment safe. Many accidents caused by defective brakes might have been avoided by frequent and thorough brake inspections. These brake inspections must be done more frequently when vehicles are used in sand, mud, or fording.
This manual discusses the basic principles associated with inspecting and troubleshooting brake systems. The purpose of this information is to give you an analytical understanding of the interrelationships of principles and components of an operating system. When you understand the operation of a system, it is much easier to analyze a malfunction and make the proper repairs.
When you have completed this manual, you will be able to:
The stopping distances for all vehicles depend on the distance the driver can see and think before pressing the brake pedal. Figure 1 shows some stopping distances from different speeds with good brakes. These stopping distances came from actual tests.
Figure 1 - Total vehicle stopping distance of an average vehicle.
Hydraulic brakes should be inspected for the external condition of the hoses and tubing, especially for leakage or seepage at the couplings. Hose or tubing worn or weakened by rubbing against other parts of the vehicle must be replaced.
Under no circumstances should steel brake tubing be replaced with copper tubing.
Test for leakage by holding the brake pedal depressed for at least 1 minute. If the pedal does not hold, there is a leak in the system. If you find a leak, repair it, even if you have to pull all the wheels to examine the wheel cylinders. Then fill the master cylinder with fluid and bleed the brakes.
Silicone brake fluid will not mix with glycol brake fluid and no adverse effects will occur to brake parts if it is combined accidently in small quantities. Some of the advantages of silicone brake fluid are that it will not damage painted surfaces, it has excellent dielectric properties, it will not deteriorate during long periods of system storage or climatic exposure, and it will not absorb or retain moisture.
To comply with requirements for testing brakes, you must see that at least one of the wheels is removed to check the brake lining and drum/rotor. Some manufacturers recommend pulling two wheels--one on each side. Look for loose or broken brake shoe retracting springs, worn clevis and cotter pins in the brake operating mechanisms, and grease or oil leaks at the wheel bearing grease retainer. Check for any signs of brake fluid leakage around the wheel cylinders or caliper operating pistons.
Brake linings that pass inspection for wear must be securely fastened to the brake shins and free from grease and oil. Small grease or oil spots can be removed from the lining with a non-oil based solvent. Linings saturated with grease or oil should be replaced only after the source of contamination has been repaired.
Badly worn or scored brake drums/rotors should be machined smooth and true on a lathe or replaced (Figure 2). Cracked brake drums or brake drums that have been machined beyond their maximum allowable diameter should be discarded. Brake drums and discs have the maximum or discard diameters cast into their outer surfaces.
Figure 2 - Checking and inspecting brake drums.
Brake shoe and drum trouble that is detected during road tests but is not immediately evident when the wheels are pulled may be caused by the wrong kind of lining, ill-fitting brake shoes, or brake drums slightly out of round. The clue to these troubles may be chattering, spongy, or grabbing brakes.
Before troubleshooting brake systems by road testing, be sure that the vehicle is mechanically sound. Different size tires, low tire pressures, faulty shock absorbers, loose wheel bearings, and worn front-end parts may each indicate brake problems where there are none.
An inexperienced mechanic may reverse the primary and secondary shoe on one of the wheels or interchange them between wheels so that the shoes are not exactly mated with the drums against which they expand. If you replace shoes or machine the drum/rotor on one side, do the same to the opposite side to prevent pull or loss of control.
The preceding paragraphs apply to most braking systems but do not list all of the problems you will have. For other probable causes of trouble and their remedies in standard hydraulic brake systems, refer to Table 1.
Table 1 - Troubleshooting Chart for Hydraulic Brakes (Standard).
Pedal reserve is the distance from the brake pedal to the floorboard with the brakes applied (Figure 3). Low or no pedal reserve indicates brake problems. When there is no pedal reserve or an unlikely occurrence with a dual master cylinder, it could mean anything from a lack of brake fluid, to worn brake linings, a faulty master cylinder, or only the need for a simple brake adjustment. Each of these conditions demands that you closely inspect the brake system.
Figure 3 - Brake pedal reserve.
Dragging brakes are caused by the following: one or more sets of shoes being adjusted too tightly, broken or weak return springs, a wheel cylinder piston that is stuck, drums that are out of round, defective lining material, loose anchor pins, or clogged lines or hoses. When both rear wheels drag, the cause may be the parking cable linkage being adjusted too tightly or a frozen parking brake cable. All wheels dragging can be the result of a stuck master cylinder pedal linkage or a defective power booster.
Be sure all other parts related to the front end are in good working order before placing blame on the brakes. Loose anchor pins or backing plates, improper lining, wrong adjustment, broken return springs, drums out of round, defective wheel cylinder, a binding disc caliper piston, or a clogged or crimped hydraulic line can all cause a vehicle to pull to one side during braking.
The most common cause for a soft or spongy brake pedal will be air trapped in the hydraulic lines. This problem may also be caused by a brake drum being cut too thin when it is being resurfaced, and by weak or old flexible brake lines.
This problem may be the result of grease or brake fluid on the lining, pedal linkage binding, a faulty master cylinder, or glazed brake linings.
Incorrect brake adjustment, or brake lines or brake lining fouled with grease or brake fluid maybe the cause of sensitive brakes.
Before you determine a noise to be coming from the brakes, eliminate all other possible sources, such as body noise, loose front-end parts, loose lug nuts, and so forth. Brake noise may be coming from shoes scraping the backing plate; in addition, loose brake lining (riveted), loose anchor pins, loose or weak return springs, and loose backing plates can all cause some sort of brake noise.
Brake fluid loss is a serious problem caused by loose fittings, or leaking wheel cylinders, master cylinder, brake lines, and hydraulic hoses.
The brake drum must be removed to check the self-adjust mechanism (Figure 4). Worn or frozen star wheels, broken or dislodged adjusting cable, or broken hold-down clips will all cause the self-adjuster to malfunction.
Figure 4 - Self-adjusting brake mechanisms.
If the brake failure warning lamp comes on, it is a signal that one of the two hydraulic circuits has malfunctioned. Check the entire system and after you make any repairs, reset the brake failure warning lamp switch (Figure 5). See Table 1 for a complete listing of possible brake problems and repairs.
Figure 5 - Pressure differential valve with brake lamp warning switch.
|Test Your Knowledge
1. Braking systems are usually inspected yearly after what maximum number of miles?
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Aside from the vacuum booster, the same basic inspection procedures given in the hydraulic brake section apply to the vacuum-assisted hydraulic brake system. When you check this system for a source of trouble, refer to the chart for the standard hydraulic brake system (Table 1). After you isolate possible causes by consulting this chart, check for causes in the troubleshooting chart of Table 2.
Conduct the following test BEFORE you check the cause of a hard pedal. With the engine stopped, depress the brake pedal several times to eliminate all vacuum from the system. Apply the brakes, and while holding the foot pressure on the brake pedal, start the engine. If the unit is operating correctly, the brake pedal will move forward when the engine vacuum power is added to the pedal pressure. If this test shows the power unit is not operating, check the probable causes of vacuum failure in Table 2 (below).
Table 2 - Troubleshooting Chart for Vacuum-Assisted Hydraulic Brakes (Power).
A "hard pedal" means the booster is inoperative, and you should suspect and check the following as the cause: collapsed vacuum hoses, faulty vacuum check valves, internal damage to the power booster, or a broken plunger stem.
Uncontrolled stopping is a problem that may be caused by grabbing or oversensitive brakes. This symptom may result from a faulty power booster, a damaged vacuum check valve, leaky or incorrectly connected vacuum lines, or a broken plunger stem.
When you apply the brakes and they fail to release, the following could be the problem- a broken power booster return spring, a sticking valve plunger in the booster, or a jammed power piston.
Loss of brake fluid may occur through the rear seal of the master cylinder past the piston stop plate and into the power booster. The leak is not visible on the backing plates, the wheels, or the frame because the fluid collects in the power booster. Some of the fluid may be drawn through the vacuum lines and burned in the engine. The end result is that you do not see the leak. For a more complete listing of vacuum booster hydraulic brake problems and remedies, see Table 2. Always consult the specific manufacturer's manual whenever you replace or repair any vacuum power booster.
Diesel engines do not create enough usable vacuum to actuate the vacuum power brake booster. The alternative is a hydraulic-assisted power brake booster or hydroboost. The hydroboost uses hydraulic pressure developed by the power steering pump rather than vacuum from the engine (Figure 6). The booster unit contains a spool valve that has an open center that controls the pump pressure as braking occurs. A lever assembly has control over the valve position and the boost piston provides the necessary force that operates the master cylinder. See Figure 7 for a parts breakdown of the booster assembly.
Figure 6 - Hydraulic power booster system.
Figure 7 - Hydraulic power booster assembly.
In the event of hydraulic pressure loss, a spring-loaded accumulator is provided on the unit. This will provide for at least two power brake applications. The brakes will operate without the power assist unit, but the pedal pressure will be noticeably higher. AVOID DRIVING IN THIS CONDITION.
Hard Pedal (at an Idle). This problem may be caused by fluid contamination, pedal linkage binding, or a bad hydroboost unit.
High Pedal and Steering Effort. A loose or broken power steering belt, low pump fluid level, low engine idle, a restriction in one or more hydraulic lines, or a defective power steering pump will cause these symptoms.
Slow Pedal Return. Slow pedal return can be caused by pedal linkage binding, a restricted booster hydraulic line, or an internal problem with the hydroboost unit.
Pedal Pulsation. Pedal chatter/pulsation is caused by a loose or slipping drive belt, low power steering fluid level, a defective power steering pump, or a defective hydroboost unit.
Brakes Too Sensitive. Pedal linkage binding or a defective hydroboost unit will cause this to happen.
Excessive Noise.Excessive noise originating in the hydroboost unit is an indication of the following problems: low power steering fluid, air in the power steering fluid, a loose power steering belt, or a restriction in the hydraulic hoses.
The interchanging of parts between hydroboost units of different makes of CESE is not recommended. Tolerances of parts and pressure differentials may be different, causing a jerry-rigged hydroboost unit to exceed the normal 1,400 psi accumulator pressure.
INJURY TO PERSONNEL AND DAMAGE TO THE VEHICLE COULD BE THE RESULT. PROTECT YOURSELF. USE THE MANUFACTURER' S SPECIFICATIONS WHEN YOU WORK ON THESE UNITS.
The manufacturer recommends that this unit not be rebuilt or overhauled. If the problem is in the booster, replace the booster.
The purpose of air braking systems is to enable the operator to apply sufficient braking action to the wheels of larger and heavier trucks and construction equipment (Figure 8). Considerable force is available for braking since the operating pressure is as high as 110 pounds per square inch. More often, stopping distances will be much greater than those shown in Figure 1, primarily because of the increased weight of the equipment and load.
Figure 8 - Typical air brake system.
When you are troubleshooting, first make a visual inspection and check all the obvious things--open air drain cocks, off-track compressor belt, broken air lines, and so forth.
Next, perform an air buildup test and an air leakage test.
Perform the air buildup test in the following sequence:
Figure 9 - Air reservoir with an air drain valve.
Conduct the air leakage test with the air brake system at normal operating pressure and the engine turned off. Hold the air brakes in the maximum applied position and watch the air pressure gauge on the dashboard of the vehicle. The air pressure should not drop more than 3 pounds in 1 minute after the brakes are applied and 2 pounds in 1 minute with the brakes released. If the indicated air pressure drops more rapidly than the times specified here, there is an air leak in the system. Trace the air lines to determine the exact source of the leak. Since air leaks normally make a distinct hissing sound, when you find the source of the noise, you have found the leak. Smaller leaks are not as audible and are harder to detect; however, you can detect these leaks by brushing the hose or tubing connections of the air brake system with a solution of soapy water. Air bubbles indicate a leak.
Air brakes on trailers get an external brake inspection as part of the inspection required on a truck-trailer combination. They are also tested for holding as if the trailer were suddenly disconnected from the tractor. To conduct this test, first make sure the air lines between the tractor and trailer are coupled properly. Then, after you start the engine so both tractor and trailer air reservoirs are charged, quickly and simultaneously disconnect both air line couplings. The trailer or semitrailer brakes should be automatically applied. Trailer brakes are designed to stop the trailer when it is accidentally disconnected from the towing vehicle. All states require automatic application of trailer brakes in an emergency. Some states go even further for trailers having a chassis and body weight of 1,000 pounds or over; such trailers must be equipped with adequate brakes that will also hold the vehicle for at least 15 minutes after application.
If these inspections and tests do not disclose the fault, consult the troubleshooting chart of Table 3.
Table 3 - Air Brake System Troubleshooting Chart.
On vehicles equipped with air-over-hydraulic brakes (Figure 10), do a good visual inspection of the air compressor, the air reservoir, the air lines, the brake pedal and linkage, the wheel brakes, the master cylinder, and the hydraulic line from the master cylinder to the air-hydraulic power cylinder and from the air-hydraulic power cylinder to the wheel brakes.
Figure 10 - Air-over-hydraulic brake system.
Operating troubles resulting from malfunction of the air-over-hydraulic power cylinder are hard pedal (excessive pedal pressure required to apply the brakes) and dragging brakes (power cylinder fails to return to released position when the brake pedal is released).
To test a sluggish or inoperative power cylinder, first install an air pressure gauge in the control valve housing and a hydraulic gauge at both the hydraulic fluid inlet line and the hydraulic brake line output port. Then slowly depress the brake pedal and observe the gauges. When the air control pressure gauge shows between 1 and 5 psi, the hydraulic pressure at the hydraulic inlet should not exceed 40 psi. Excessive hydraulic pressure indicates a sticking relay piston (caused by swollen or damaged piston scaling cups or a corroded or damaged relay piston sleeve) or sticking control valve poppets (caused by corrosion of the poppets, poppet seats, or damaged poppets).
With the brake pedal completely depressed in the fully applied position, the air control pressure gauge should show 90 psi and the hydraulic output pressure gauge should show full power (runout) pressure of 1,400 to 1,600 psi. Low pressure or no pressure on the air pressure gauge indicates air leakage or an inoperative control valve. Low hydraulic output pressure indicates hydraulic fluid leakage, a sticking hydraulic piston, or an inoperative check valve (in the hydraulic piston), or a residual line check valve.
To test for internal and external air leakage or hydraulic leakage, fast depress the brake pedal and apply soapsuds at the air control line and its connections, the double check valve (if so equipped), and the cylinder body and end plate. Bubbles appearing at any of these points indicate external air leaks. While the pedal is depressed, check for hydraulic fluid leakage at the outlet fitting cap and around the jam nut on the slave cylinder housing. Internal air leakage is indicated by a pressure drop in excess of 2 psi in 15 seconds. The trouble is a worn or damaged piston packing, a scored cylinder body, or leakage at the poppets in the control valve. Internal hydraulic pressure leakage can also be indicated by hydraulic pressure drop at both hydraulic pressure gauges while the brake pedal is depressed.
Dragging brakes can be tested by releasing the brake pedal and observing the air pressure gauge and the two hydraulic pressure gauges. All gauges should register zero without lagging. When pressure is noted at the air pressure gauge, a sticking relay piston, damaged or corroded control valve poppet, or a ruptured control valve diaphragm exists. Pressure at the hydraulic pressure gauges indicates a sticking hydraulic piston, a sticking power piston, or a weak or broken piston return spring. If the hydraulic pressure gauges show a slow pressure drop, it indicates a defective check valve (in the hydraulic piston) or a defective residual line check valve.
If the tests indicate external air leakage, tighten the control line connections, and/or replace a damaged control line or control line gasket, or double check the valve. For internal air leakage you must remove the unit to replace worn or damaged power piston packing or end plate gasket, and repair or replace the cylinder body or end plate.
If the tests indicate hydraulic fluid leakage, an inoperative control valve, sticking power piston, relay piston, or hydraulic piston, remove the unit for disassembly and repair or replace the worn or damaged parts.
Serviceable parking/emergency brakes are essential to the safe operation of any piece of automotive or construction equipment. Several types of these brakes are manufactured, such as the external contraction, drum, and disk types (Figure 11). These are drive line brakes common to heavy construction equipment, and are usually mounted on the output shaft of the transmission or transfer case directly in the drive line. Theoretically, this type of system is preferred for heavy equipment because the braking force is multiplied through the drive line by the final drive ratio, and the braking action is equalized perfectly through the differential. Drawbacks are that severe strain is placed on the power transmission system, and also that the vehicle may move while it is being lifted since the differential is not locked out.
Figure 11 - Transmission mounted emergency/parking brake.
Parking brakes interconnected with service brakes are usually found on automotive types of equipment (Figure 12). This type of emergency/parking brake is actuated by a foot pedal or a dash mounted handle assembly, and is connected through linkage to an equalizer lever (Figure 13), rod assembly, and cables connected to the emergency/parking brake mechanism within the drums/discs at the rear wheels.
Figure 12 - Automotive parking/emergency brake.
Figure 13 - Equalizer linkage.
When you test parking brakes, stop the vehicle on a road graded at about 30 percent. Set the parking brake and release the service brakes. The vehicle should maintain its position and not roll or inch backwards. Repeat the test in the opposite direction. Again, the vehicle should hold its position. If there is no hill close by, you may test parking brakes by setting the brake, placing the vehicle in first gear (low), and slowly releasing the clutch with the engine idling (do not rev the engine while doing this exercise). This action should stall the engine of the vehicle you are testing. In the case of an automatic transmission, the vehicle should not move in any gear. In either of these tests, if the vehicle does move, it is an indication that there is a parking brake malfunction.
Once you determine there is a problem, proceed as follows. First, inspect the condition of the emergency brake linings and contact surfaces just as you would for service brakes and just as carefully. Pay attention to the ratchet and pawl or any other automatic locking device that holds the brake in the applied position to make sure it is operating properly. In addition, when inspecting the drive line type brake, examine the universal joints and splines for loose bolts and grease leaks. Loose bolts are not uncommon for vehicles having brakes mounted in the drive line.
The emergency brake must hold the vehicle on any grade. This requirement covers both passenger and commercial motor vehicles equipped with either the enclosed type of emergency brake at each rear wheel or a single emergency brake mounted on the drive line. The Federal Motor Carrier Safety Regulations Pocketbook lists emergency brake requirements.
The HMMWV has one of two types of parking brakes. It is equipped with either a single parking brake assembly or a left and right parking/service brake. They need to be inspected semi-annually, and you must replace the pads when they are 1/8 inch in thickness or less.
On versions equipped with the single parking brake assembly, the system works much like the ones discussed earlier that are located on the transmission shaft. There is a separate parking brake located between the rear driveshaft and the rear differential. The pads are squeezed against the rotor when the hand brake is applied.
On versions that have a service/parking brake in one system, the parking brake is applied through a mechanical linkage when the parking brake lever is set.
Use Silicone Brake Fluid (BFS) for filling master brake cylinder. Failure to use BFS will cause damage to brake cylinder.
Vehicles that operate air brake systems have a different type of parking brake. The dual chamber air brake system also uses a spring inside the chamber. This spring is held off with air pressure. When air pressure is applied, the diaphragm pushes the spring back and releases the brakes. When there is a decrease of air pressure, the spring takes over and pushes against the brakes locking the wheel.
|Test Your Knowledge
2. A standard power booster will not work with a diesel engine for which reason?
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The first anti-lock brake systems (ABSs) were developed and used in aircraft in the early 1950s. Certain automobiles had the systems in the experimental stages in the mid 1950s and in the production stages in the early 1970s. The ABSs are common today in many production cars and trucks.
Why we use ABS is simple ... CONTROL. A high percentage of vehicle accidents on the highway are caused by skidding. Since braking is most effective and steering is not lost when the wheels are still rotating, the anti-lock brake system prevents skidding by allowing the wheels to continue turning during maximum braking effort. On wet pavement, hydroplaning of the tires is cut to a minimum. One final benefit is that of extended tire wear by the elimination of flat spots caused by brake lockup during panic stops.
All ABS (either two wheel or four wheel) operate on the same principle, that is, the system is monitored by an electronic control module for the rate of reduction of vehicle wheel speed during brake system operation. If the system feels that lockup is about to occur at one or more wheels, modulated hydraulic pressure is fed to that brake caliper by a hydraulic control unit or an electro-hydraulic valve. In this way, even if hydraulic pressure is not the same at each wheel, maximum tire adhesion to the road surface is maintained. Once again, the way the modulated hydraulic pressure is maintained is different with each manufacturer. Before going any further, get a copy of the manufacturer's maintenance and repair manual of the vehicle that you are working on.
The first equipment you are most likely to see the system used on is automotive type CESE. Very little should malfunction on the system. If the ABS is in need of repair, you should take the following precautions before working on it:
For further reading concerning anti-lock braking systems, consult the manufacturer's service and repair manual of the vehicle you are working on.
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In this manual you learned how to inspect hydraulic, power, and air brake systems. In addition, you learned troubleshooting techniques for those systems as well as some safety protocols. The ability to stop a vehicle that is heavy or moving at great speeds is one of the most important responsibilities of a construction mechanic. After all, there is no known way to measure how many lives you can save by possessing this unique ability. When you have mastered the knowledge of these systems, you will become a better CM.
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1. Under what circumstances would copper tubing be used in a brake system?
2. When testing for leakage in a hydraulic brake system, you must depress and hold the brake pedal for at least how many minutes?
3. NFELC maintenance bulletin #75 directs the Naval Construction Force to use which fluid or materials?
4. Brake drums that have been worn or machined past their discard diameter or thickness must not be used.
5. Which condition could indicate brake problems where none, in fact, exist?
6. Which statement provides a good description of pedal reserve?
7. Both rear brakes may drag as a result of which problem?
8. A brake drum that is cut too thin will cause which problem?
9. After completing repairs to a brake system, you should take which action first?
10. On a power brake system with a vacuum booster, if the air valve sticks, what will happen?
11. In a brake system that uses a vacuum booster, a hard pedal could indicate which situation?
12. In a brake system using a vacuum booster, a hydraulic leak may not be seen for which reason?
13. On a vehicle using a hydroboost power brake system, hydraulic pressure is created by which means?
14. In the event of a hydroboost power brake system failure, the spring-loaded accumulator will provide for a total of how many power brake applications?
15. When the power steering belt breaks in a hydroboost power brake system, which situation will occur?
16. Excessive noise in a hydroboost power brake system could be caused by which problem?
17. What is the normal accumulator pressure of a hydroboost power brake system, in psi?
18. The stopping distance of construction equipment and heavy trucks is greater due to which factor?
19. An air brake system should build up to safe operating pressure in what maximum number of minutes?
20. You should check for inaudible air leaks by which means?
21. The automatic application trailer brakes must hold a vehicle for how many minutes?
22. In an air-over-hydraulic power braking cylinder, excessive hydraulic pressure would likely be caused by which part?
23. In an air-over-hydraulic power braking cylinder, internal air leakage is considered excessive if there is a pressure drop of 2 psi in what number of seconds?
24. On construction equipment, the drive line brakes are usually mounted in which location?
25. When compared to an emergency braking system that is interconnected with the rear service brakes, a drive line emergency braking system has greater holding power for what reason?
26. A parking brake that is interconnected with the service brake is usually found on what type of equipment?
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