MD CDL Air Brakes cheat sheet

The compressor builds air pressure; the governor controls when the compressor loads and unloads, typically around 100-125 psi.

The air dryer removes moisture and contaminants from compressed air before it enters the wet tank; it has a purge cycle and a replaceable desiccant cartridge.

The wet tank receives air directly from the compressor and supplies the dry tanks; it collects moisture and is drained daily.

The supply (dry) tank stores air for the brake system; it is protected by a one-way check valve so pressure is not lost if the wet tank fails.

The brake pedal (foot valve) controls the amount of air pressure applied to the brake chambers; it is a graduated control, not an on/off switch.

The slack adjuster converts the linear pushrod stroke from the brake chamber into rotary motion to apply the foundation brake; it also provides a mechanical advantage.

The brake chamber converts air pressure into mechanical force via a diaphragm and pushrod; a spring brake chamber contains a heavy spring for parking and emergency braking.

The quick release valve speeds up release of air from the brake chambers when the pedal is released, reducing lag time.

The relay valve in a tractor protection system speeds up application and release of the trailer brakes by using a small control signal to open a larger air passage.

The compressor is engine-driven and typically belt- or gear-driven; it must build air pressure from 85 to 100 psi within 45 seconds at fast idle with the engine at operating RPM.

The governor cuts in (loads the compressor) at about 85 psi and cuts out (unloads) at about 100-125 psi; the cut-out pressure must not exceed the safety valve setting.

The safety valve (pop-off valve) is set to open at about 150 psi to protect the system from over-pressurization; it is not a regulator.

The wet tank must be drained daily to remove moisture and oil; the dry tank(s) should also be drained regularly.

The air dryer purge valve expels moisture and oil during the purge cycle; a faulty dryer can cause moisture to reach the brake system and freeze in cold weather.

The supply reservoir must be protected by a one-way check valve so that loss of pressure in the wet tank does not deplete the dry tank.

The low air pressure warning must activate before the pressure drops below 60 psi; the warning is typically a red light and buzzer.

The spring brakes will begin to apply automatically when air pressure drops to approximately 45 psi (the spring brake priority valve setting).

The brake pedal is graduated: pressing harder increases air pressure to the brake chambers, producing more braking force; releasing the pedal reduces pressure and releases the brakes.

A lag time exists between pedal movement and brake application, especially in long combination vehicles; the driver must anticipate this delay.

The pedal should be pressed firmly and steadily; pumping the pedal can reduce air pressure and is not a normal braking technique.

In a tractor-trailer, the foot valve first applies the tractor brakes, then the trailer brakes via the relay valve; the trailer brakes may apply slightly after the tractor brakes.

The brake pedal must not be used as a parking brake; it will not hold the vehicle and air pressure can leak off.

When the pedal is released, air from the brake chambers is exhausted through the foot valve or quick release valves; a slow release indicates a restriction or faulty valve.

The foot valve is also called the application valve; it controls pressure to the primary and secondary circuits.

If the pedal feels spongy or goes to the floor, there is a leak or air in the system; the vehicle must be taken out of service.

Before driving, check that the air pressure builds from 85 to 100 psi within 45 seconds at fast idle; if not, the compressor or governor may be faulty.

With the engine off and the system fully charged, the air pressure should not drop more than 3 psi in one minute for a single vehicle, or 4 psi for a combination vehicle (leak test).

With the engine running and the governor cut out, apply the brakes fully and hold; the pressure should not drop more than 3 psi in one minute (applied leak test).

Check the low air pressure warning: it must activate before the pressure drops below 60 psi; pump the brakes to reduce pressure and observe the warning.

Check the spring brakes: with the engine off, release the parking brake and fan the brakes; the spring brakes should apply when pressure drops to about 45 psi.

Check the slack adjusters: with the brakes released, pull the slack adjuster by hand; it should not move more than about 1 inch (25 mm) at the pushrod.

Check the brake chambers for cracks, dents, or leaks; check hoses for chafing and leaks; check the air dryer for proper operation.

Perform the tug test: with the trailer brakes applied, gently pull the tractor forward to ensure the trailer is securely coupled and brakes hold.

The low air pressure warning must be a red light and a buzzer (or other audible alarm) that activates before the pressure falls below 60 psi.

If the warning activates while driving, the driver must immediately move to a safe location and stop; the spring brakes will apply automatically at about 45 psi.

The spring brakes are the emergency brake system; they are held off by air pressure and apply by spring force when pressure is lost.

The tractor protection valve closes automatically when pressure drops to about 45 psi, preventing loss of air from the tractor and applying the trailer brakes.

The parking brake control (yellow diamond knob) controls the spring brakes; pulling it out applies the spring brakes, pushing it in releases them (if air pressure is sufficient).

In an emergency, if the service brakes fail, the driver can use the spring brakes by pulling the parking brake control; they should be applied gradually to avoid skidding.

The emergency relay valve on the trailer provides emergency braking if the trailer air supply is lost.

A burst diaphragm or failed brake chamber will cause a continuous air leak and may trigger the low pressure warning; the vehicle must be stopped and repaired.

The parking brake is typically the spring brake, applied by pulling the yellow diamond-shaped knob; it must be fully applied when parking.

The spring brakes will not release unless air pressure is at least 45-60 psi; if pressure is too low, the vehicle cannot be moved until pressure builds.

To make an emergency stop with spring brakes, pull the parking brake knob gradually; sudden application can cause the wheels to lock and skid.

If the service brakes fail, downshift and use the spring brakes as a backup; do not rely on engine braking alone.

When parking on a grade, apply the parking brake, place the transmission in a low gear (or park for automatic), and use wheel chocks if necessary.

The parking brake must hold the vehicle stationary on a grade; if it does not, the brakes need adjustment or repair.

In a combination vehicle, the parking brake control applies the tractor spring brakes; the trailer spring brakes are applied by the trailer parking brake control or automatically when air is lost.

After an emergency stop using spring brakes, the system must be inspected and the cause of air loss repaired before driving.

Air brakes have a lag time; maintain a longer following distance than with hydraulic brakes, especially in a combination vehicle.

Use the brakes smoothly and progressively; avoid pumping the pedal, which wastes air and can cause a loss of pressure.

On long downgrades, use engine braking (downshift) and stab braking (apply brakes firmly, release when speed drops, repeat) to control speed without overheating the brakes.

Do not ride the brakes; continuous application can cause brake fade and overheating, reducing braking effectiveness.

In cold weather, drain air tanks daily to prevent moisture from freezing in lines and valves; use alcohol evaporators if equipped.

Before driving, always perform a pre-trip inspection including the air brake tests; a faulty air brake system is a serious safety hazard.

When stopping, allow extra distance because air brakes take longer to apply than hydraulic brakes; anticipate stops.

If the low air pressure warning activates, stop immediately in a safe place; do not continue driving.