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Learn safety requirements, maintenance, troubleshooting, and common procedures in air brake system servicing. Understand brake valve failures, foundation brakes overhaul, and more.
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Chapter 31 Air Brake Servicing
Objectives (1 of 3) • Understand the safety requirements of working on an air brake system. • Perform basic maintenance on an air brake system. • Diagnose common compressor problems. • Describe the procedure required to service an air dryer. • Performance test an air dryer.
Objectives (2 of 3) • Check out the service brakes on a truck. • Test the emergency and parking brake systems. • Verify the operation of the trailer brakes. • Understand the OOS criteria used by safety inspection officers. • Diagnose some brake valve failures. • Describe the procedure required to overhaul foundation brakes.
Objectives (3 of 3) • Determine brake free-stroke and identify when an adjustment is required. • Outline some common service procedures used on air disc brake systems.
Shop Talk • When replacing brake hose and fittings, be sure to use the same sizes, or the movement of air through the system will be altered. • One air brake OEM has determined that replacing a straight pipe nipple with a 90-degree elbow is equivalent to adding 7 feet of brake hose into the circuit. Adding a 45-degree elbow is equivalent to adding 3 feet of hose into the circuit.
Caution • All air tanks that are FMVSS 121-compliant must have a means of mechanically draining them. • Most automatic drain valves are equipped with a mechanical dump valve, but it can be difficult to locate. One design uses a Schrader-type plunger that is difficult to access.
Shop Talk • Maximum allowable leakage/drop-off rates are often defined by local jurisdictions (state and provincial governments). • These regulations may differ from the test values used here. Contact your local transportation enforcement office to identify the specifications used in your area.
Caution • All compressors pass minute quantities of oil into the air circuit. • Avoid replacing a compressor that is passing trace oil because the warranty will usually be denied by the OEM. • The warranty also will be denied if the drain-back ports are plugged with silicone.
Compressor Problems • See Table 31-1 on page 1010 of the textbook.
Shop Talk • Carefully label and code every line as it is removed from the foot valve assembly. • Failure to do this can considerably lengthen the time required to install the replacement valve.
Warning • Always cage the compression spring with the release tool. • Never rely on air pressure to keep the spring compressed.
Caution • Never open a disarmament chamber until the main spring has been observed through the inspection windows to be separated from the housing and is not under tension.
Caution • The loaded main spring in a spring brake assembly contains a potentially lethal force. • Manual caging devices must be used whenever the spring chamber assembly is removed and replaced from the foundation brake mounting plate. • Most current spring brake units are sealed with a band clamp that has to be destroyed to separate the spring chamber in order to prevent the unit from being disassembled.
Caution • The clevis must be installed in the correct position on the pushrod, or the slack adjuster will not adjust the brake correctly.
Shop Talk • The adjusting pawl assembly can be on either side of the housing or on the front of the housing. • Make sure that the pawl assembly can be removed after the slack adjuster is installed. • The pawl assembly must sometimes be removed when the slack adjuster is serviced.
Caution • If the pawl is not removed, the teeth will be damaged when the manual adjusting nut is turned.
Air Valve Troubleshooting Guide • See Table 31-2 on page 1029 of the textbook.
Caution • On 16.5-inch brake shoe foundation assemblies using 1-inch anchor pins, the spider anchor pin bore must not exceed 1.009 inches (25.63 mm).
Shop Talk • Used drums are almost never machined and returned to service today. • This is because of the low cost of new replacement drums when compared to the labor required to turn a drum. • Additionally, machining used brake drums is technically not a good practice, as the drums have a much shorter in-service life because of much harder friction linings.
Caution • Cast drums are machined in manufacture. However, when stored, especially for long periods, they can deform and lose their concentricity. • It is, therefore, a good practice to machine brand new drums to ensure that they are perfectly concentric with the wheel.
Shop Talk • Modern friction facings on brake shoes are much harder than the asbestos base linings used on drums a generation ago. • They have a longer service life. • The result is that the life of the linings can be the same as the life of the drums in some applications. • It is almost impossible to machine/turn down the effects of severe heat checking on a brake drum because the drum becomes so hardened. Cast brake drums are cheap when compared with the labor cost required to turn a drum. Many operations routinely replace brake drums today when the brakes are relined.
