MS CDL Air Brakes cheat sheet

A compressor, driven by the engine, builds air pressure; a governor controls when the compressor pumps (cut-in about 100 psi) and when it stops (cut-out about 120-125 psi).

Air flows from the compressor through a supply line to the reservoir tanks; a safety valve on the supply tank releases at about 150 psi to prevent over-pressurization.

The foot valve (brake valve) controls the amount of air pressure applied to the brakes; the harder the pedal is pressed, the higher the delivered pressure.

The slack adjuster converts pushrod stroke into brake shoe movement; its length gives a mechanical advantage to the S-cam.

The S-cam rotates to spread the brake shoes against the brake drum; the pushrod stroke length is what determines whether adjustment is correct.

A low-pressure warning signal (buzzer and/or light) must activate before pressure drops below 60 psi; some vehicles also have a wig-wag mechanical flag.

On a dual-circuit system, the supply reservoir feeds the primary (rear) circuit through a one-way check valve, and the secondary (front) circuit is fed separately.

The alcohol evaporator adds alcohol to the air system to prevent moisture from freezing in valves and lines during cold weather.

The dual air brake system has two separate circuits: the primary circuit operates the rear brakes and the secondary circuit operates the front brakes.

A one-way check valve between the circuits prevents air from flowing back from one circuit to the other, so a leak in one circuit does not drain the other.

If one circuit fails, the other circuit still provides braking, but stopping distance increases and the low-pressure warning activates.

The parking (spring) brake is applied by heavy springs when air pressure is released; it requires air pressure to be released, so it is the failsafe brake.

The spring brakes apply automatically when system pressure drops to roughly 20-45 psi, depending on the vehicle.

The tractor protection valve closes and prevents air loss from the tractor if the trailer breaks away or the trailer air supply line separates; it also protects the tractor if trailer pressure drops.

The trailer air supply valve (red knob) controls air to the trailer; pulling it out (or releasing it) applies the trailer spring brakes.

The dash low-pressure warning must come on before pressure drops below 60 psi; a vehicle with a dual system must have both a warning light and a buzzer.

Before starting, check that the parking brake is set and wheel chocks are in place; then check that the warning buzzer and light activate when the ignition is turned on.

Build air pressure to governor cut-out (about 120-125 psi) and observe that the compressor stops pumping; then check that the pressure does not drop faster than the allowed leak rate with the engine off and brakes released.

With the engine off and the brakes released, pressure loss should be no more than 2 psi in one minute for a single vehicle, or 3 psi in one minute for a combination vehicle.

With the brakes fully applied and the engine off, pressure loss should be no more than 3 psi in one minute for a single vehicle, or 4 psi in one minute for a combination vehicle.

Check the slack adjuster pushrod stroke against the manufacturer's limit; a stroke beyond the limit means the brakes are out of adjustment and the vehicle must not be driven.

Test the parking brake by pulling the yellow knob with the engine running and pressure built; the vehicle should not move when you try to pull forward.

Test the service brakes by releasing the parking brake and moving forward slowly while applying the brakes; the vehicle should stop smoothly without pulling to one side.

Check the low-pressure warning by pumping the brakes with the engine off; the warning buzzer and light must activate before pressure falls below 60 psi.

Static leak test: with the engine off, brakes released, and pressure at cut-out, watch the gauge for one minute; loss must not exceed 2 psi (single) or 3 psi (combination).

Applied leak test: with the engine off and the brakes fully applied, watch the gauge for one minute; loss must not exceed 3 psi (single) or 4 psi (combination).

A leak in the supply side (compressor to tank) usually shows as the compressor running constantly or cut-out pressure not being reached.

A leak in the service side (foot valve to brake chambers) shows as pressure dropping when the brakes are applied and staying applied.

To locate a leak, listen for escaping air and use soapy water on fittings, hoses, and valves; bubbles indicate the leak point.

The governor cut-in and cut-out pressures should be checked; if cut-out is too high, the safety valve may release; if cut-in is too low, braking may be delayed.

Check that the air dryer purges moisture at cut-out; a constant purge or moisture in the tanks indicates a dryer problem.

Drain the air tanks daily to remove moisture and oil; a large amount of oil in the tank can indicate compressor problems.

Air brakes require a longer stopping distance than hydraulic brakes because of lag time; the brake pedal must be pressed firmly and held, not pumped.

Use engine braking (downshifting) on long downgrades to control speed; do not ride the brakes, which can overheat and fade the brakes.

The proper braking technique on a downgrade is to select a low gear before starting down and use the brakes only to control speed, releasing them to cool.

When stopping on a hill, apply the parking brake and use wheel chocks; do not rely on air pressure alone to hold the vehicle.

Air pressure must be maintained while driving; if the low-pressure warning activates, stop as soon as it is safe and investigate before continuing.

The brake pedal should not be pumped; pumping releases air pressure and can reduce braking effectiveness on a long downgrade.

When the brakes are applied, the slack adjusters move the pushrods; if the pushrod stroke is too long, braking force is reduced and the brakes must be adjusted.

In cold weather, keep the air system dry and use the alcohol evaporator; moisture can freeze in valves and cause brake failure.

If the service brakes fail, use the parking (spring) brakes by pulling the yellow knob; apply them gradually to avoid locking the wheels and skidding.

If the spring brakes are applied gradually, the vehicle will slow and stop; if applied suddenly, the wheels may lock and the vehicle may skid.

If air pressure is lost, the spring brakes apply automatically at about 20-45 psi; the driver should steer to a safe location and stop.

On a downgrade with brake failure, downshift to a lower gear to use engine braking, and use the parking brake gradually; avoid sudden application.

If the brakes fail and the vehicle is on a level road, steer to the shoulder, apply the parking brake gradually, and use any available escape ramp.

The emergency stop procedure includes turning on hazard lights, warning other drivers, and steering to the safest available location.

If a tire blows out, hold the steering wheel firmly, avoid hard braking, and slow gradually before pulling off the road.

After any brake failure, do not continue driving until the system has been inspected and repaired by a qualified mechanic.