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.

Brake Torque Balance

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.

Air Tanks

Air Brake Test Equipment

Double Gladhand (1 of 2)

Double Gladhand (2 of 2)

Governor

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.

Dash Control Valve Module

Push-Pull Valve

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.

Air Compressor

Compressor Problems See Table 31-1 on page 1010 of the textbook.

Air Drier

Typical Foot Valve

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.

Typical Relay Valve

Quick Release Valve

Warning Always cage the compression spring with the release tool. Never rely on air pressure to keep the spring compressed.

Air Chamber Release Tool

Air Chamber Service (1 of 3)

Air Chamber Service (2 of 3)

Air Chamber Service (3 of 3)

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.

Pushrod Length (1 of 5)

Pushrod Length (2 of 5)

Pushrod Length (3 of 5)

Pushrod Length (4 of 5)

Pushrod Length (5 of 5)

Slack Adjuster

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.

Shoe Replacement (1 of 5)

Shoe Replacement (2 of 5)

Shoe Replacement (3 of 5)

Shoe Replacement (4 of 5)

Shoe Replacement (5 of 5)

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.

Install the S-cam and Shoe Left and Right Side Orientation

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.

Shop Talk When an axle wheel seal fails, oil saturates the brake friction linings, which rapidly causes them to glaze, lowering their coefficient of friction or aggressiveness. When this occurs, pressure or steam washing removes only the surface oil, so the linings should be replaced. Also replace the linings on the other end of the axle to maintain brake torque balance over the axle.

Shop Talk Take some time to ensure that automatic slack adjusters are properly set up on installation. It takes much longer to manually adjust automatic slack adjusters than manual slack adjusters.

Shop Talk Avoid lubricating the cam roller face. Lubricate the cam roller bearings. These are designed to grab at their faces and rotate on their bearings in operation.

Shop Talk Out-of-adjustment brakes affect all vehicle braking, including emergency braking. Never take chances with brake adjustment. Maladjusted brakes can have lethal consequences, and every technician has a stake in the vehicles on which they work.

Shop Talk Avoid lubricating the cam roller face. Lubricate the cam roller bearings. These are designed to grab at their faces and rotate on their bearings in operation.

Shop Talk Out-of-adjustment brakes affect all vehicle braking, including emergency braking. Never take chances with brake adjustment. Maladjusted brakes can have lethal consequences, and every technician has a stake in the vehicles on which they work.

Checking Free-stroke

Shop Talk Brake freestroke is typically specified to be between 3/8 (0.375 inch) and 5/8 (0.625 inch) regardless of chamber size. Check the OEM specifications.

Air Disc Brakes

Checking Rotor Runout

Summary (1 of 3) Brake system maintenance and service procedures attempt to ensure that air-brake-equipped vehicles meet FMVSS 121 standards when in service. Brakes must meet their original standards in terms of pneumatic timing and brake torque balance. Pneumatic timing refers to the management of the air in the system to ensure that the air reaches each actuator at the correct time so that the brakes over one wheel are not applied before or after those on another wheel.

Summary (2 of 3) Brake torque balance refers to the requirement that the same amount of mechanical force be delivered to the wheels on each axle and that the brakes over all the axles on a vehicle work together to bring the vehicle to a stop. Disc brake assemblies are increasingly used in truck applications despite higher costs because of their superior brake performance. Service procedures required on disc brake assemblies require you to be able measure lining wear, lubricate calipers, and measure rotor runout.

Summary (3 of 3) Minimum training requirements and qualifications are necessary for employees who re-inspect, repair, maintain, adjust, or periodically inspect the brakes on commercial motor vehicles (CMVs). Roller brake dynamometers and plate testers are used in brake inspection testing.