Michigan CDL Air Brakes cheat sheet

A basic air brake system consists of a compressor, air storage tanks, a foot valve (brake valve), brake chambers, slack adjusters, and foundation brakes at each wheel.

The engine-driven compressor builds air pressure and is governed by a governor that cuts the compressor in at about 85 psi and out at about 100-110 psi on most systems.

The supply (wet) tank is the first tank after the compressor; it collects moisture and oil so downstream tanks stay cleaner, and it holds the supply valve for the rest of the system.

A safety valve on the supply tank releases air at about 150 psi to protect the system from over-pressurization if the governor fails.

The foot valve is a graduated, self-lapping valve: the driver can apply and hold any pressure, and releasing the treadle releases air until brakes fully release.

The brake chamber converts air pressure into mechanical force; a Type 30 chamber has about 30 square inches of effective diaphragm area, so 100 psi produces roughly 3,000 pounds of force.

A spring brake (parking/emergency chamber) uses a heavy coil spring to apply the brake when air pressure is lost; air pressure must be present to release it.

An alcohol evaporator or air dryer removes moisture from the system; air dryers with desiccant are common on modern trucks and purge automatically.

The low-pressure warning device (buzzer/light) must activate before pressure drops below 60 psi, giving the driver advance warning of a failing system.

During the pre-trip inspection, check that the low-pressure warning activates before pressure falls below 60 psi and that pressure builds from 85 to 100 psi within 45 seconds with the engine at operating rpm.

With the engine off and the system fully charged, the static leak test allows no more than 3 psi loss in one minute for a single vehicle and no more than 4 psi for a combination (tractor-trailer).

To test the applied leak rate, hold a firm brake application with the engine off; leakage must not exceed 4 psi per minute for a single vehicle or 6 psi per minute for a combination.

The tractor protection valve should close and the trailer brakes should set automatically when air pressure drops to about 20-45 psi, preventing a runaway trailer.

Check slack adjusters by pulling them by hand: normal free play is about 3/8 to 1/2 inch at the push rod, and excessive free play signals the need for adjustment or service.

Push rod travel beyond the adjustment limit (commonly 2 inches for a Type 30 chamber, less for smaller chambers) indicates brakes out of adjustment and the vehicle must be placed out of service.

Inspect brake chambers, hoses, and lines for cracks, chafing, and leaks; soapy water applied to fittings reveals bubbles at leaking connections.

Drain the air tanks daily to remove moisture and oil; on vehicles with automatic drain valves, verify they operate and manually drain the supply tank periodically.

Check the air compressor belt tension and condition, and confirm the air dryer purge valve operates; a failed dryer lets moisture reach the tanks and can cause winter freeze-ups.

If the low-pressure warning sounds while driving, stop at the first safe place, because brake application pressure may be lost; continued driving risks total brake failure.

A rapid loss of air pressure with a loud hiss usually indicates a burst line or failed chamber; apply the brakes firmly and stop, then secure the vehicle with the parking brake.

Spring brakes apply automatically when system pressure drops to about 20-45 psi; this is the emergency stopping mechanism, so the vehicle will not roll freely at low pressure.

If the spring brakes apply on a highway, the vehicle will stop in a straight line if steering is held steady; do not release the parking brake control until the vehicle is stopped and secure.

If the foot valve fails and brakes will not apply, use the trailer hand valve (if equipped) or the parking/emergency control to slow and stop, and use engine braking and downshifting.

A compressor failure is indicated by pressure that will not build or builds very slowly; stop driving because the remaining air supply will be exhausted by repeated brake applications.

If the air dryer freezes in winter, moisture can block lines and cause brake drag or failure; alcohol evaporators or heated dryers help, and tanks should be drained before cold weather.

When brakes overheat (fade) from long downgrades, stop and let them cool; do not set the parking brake on hot drums, which can warp them or cause the linings to crack.

The three-part air brake check: (1) with the engine off and system charged, check for static leaks; (2) with the engine running, check the low-pressure warning and compressor build-up; (3) with the engine off, apply and hold the brakes to check applied leakage.

Compressor build-up test: with the engine at fast idle and pressure at 85 psi, the system should reach 100 psi within 45 seconds on a typical vehicle.

Low-pressure warning test: with the engine off, pump the brake pedal to reduce pressure; the warning buzzer/light must come on before pressure drops below 60 psi.

Static leak test: charge the system to governor cut-out, shut off the engine, wait one minute, and record pressure loss; more than 3 psi (single) or 4 psi (combination) requires repair before driving.

Applied leak test: with the engine off and full pressure, hold a firm brake application for one minute; loss must not exceed 4 psi (single) or 6 psi (combination).

The parking brake (spring brake) test: with the system charged, set the parking brake and try to move; the vehicle must not move, confirming the spring brakes hold.

On a downgrade, select a gear low enough that the engine governor and brakes are not needed to hold speed; use the same gear going down that would be needed to climb the grade.

Before driving, perform the walk-around inspection and the air brake check in the order specified by the CDL manual: leak check, warning device, build-up, and applied leak check.