MT CDL Air Brakes cheat sheet

The compressor, engine-driven and usually gear- or belt-powered, builds and maintains the system air supply; a governor cuts it out near 120-125 psi and cuts it back in near 100 psi.

The air dryer sits between compressor and reservoirs and removes moisture and oil vapor; a saturated or failed dryer sends water downstream and is a common cause of winter freeze-ups.

The supply (wet) reservoir is the first tank after the dryer and collects condensate; the primary and secondary reservoirs feed the rear and front brake circuits respectively.

A safety valve on the supply tank protects the system, typically opening near 150 psi; repeated venting means governor or compressor unloader failure.

The foot valve (treadle) is a graduated control valve, not a simple on-off switch: pedal position meters air pressure to the brake chambers.

The slack adjuster converts pushrod stroke into brake shoe movement and multiplies force; automatic slack adjusters must still be checked for free stroke and correct adjustment.

Brake chambers convert air pressure into mechanical pushrod force; a Type 30 chamber has about 30 square inches of effective piston area, so 100 psi yields roughly 3,000 lb of force.

The low pressure warning device must activate before pressure drops below 60 psi, and the system must have enough reserve to allow at least one full brake application after it sounds.

A dual (split) system has two separate circuits, typically front axle and rear axle(s); a failure in one circuit leaves the other circuit able to stop the vehicle.

The dash gauge on a dual system has two needles; both should build pressure together, and a lagging needle indicates a problem in that circuit.

The low pressure warning light and buzzer must come on before pressure falls below 60 psi and stay on until pressure is restored above the cut-in point.

The spring (parking) brake chambers are held applied by heavy springs and released by air pressure; loss of air pressure causes them to apply automatically.

On a dual system, if one circuit loses pressure the spring brakes will still apply when overall pressure drops, giving a fail-safe stopping capability.

The tractor protection valve closes to keep the tractor's air supply from being lost through a trailer breakaway; it also controls trailer air supply and emergency line pressure.

The trailer emergency (supply) line charges the trailer reservoirs; a breakaway or disconnected line triggers the trailer spring brakes to apply.

The front brake limiting (quick release) valve on some older vehicles reduces front braking to prevent lockup; it must not be confused with a functioning proportioning system.

Static leak test: with engine off, fully charged system, and brakes released, pressure should not drop more than 3 psi in one minute (single vehicle) or 3 psi in one minute for a tractor-trailer combination.

Applied leak test: with brakes fully applied and held, pressure should not drop more than 4 psi in one minute for a single vehicle; a larger drop indicates a leak in the applied circuit.

Pressure buildup test: with engine at operating rpm, the system should build from 85 to 100 psi in about 45 seconds for a single vehicle; slower buildup points to compressor, governor, or leak problems.

Low pressure warning test: pump the brakes with the engine off and confirm the warning light and buzzer activate before pressure drops below 60 psi.

Spring brake test: with pressure above 100 psi, release the parking brake, then drain pressure and confirm the spring brakes apply automatically as pressure falls.

Check the air dryer and drain all reservoirs (supply, primary, secondary) at the start of the day; significant water or oil in the drain indicates a maintenance problem.

Inspect slack adjusters, pushrods, chambers, hoses, and fittings for cracks, wear, and leaks; a pushrod stroke beyond the adjustment limit puts the vehicle out of service.

Confirm the compressor builds pressure and the governor cuts out at the correct pressure, and that the safety valve does not vent during normal operation.

Air brakes have a lag: pressure must travel through lines and chambers before force reaches the shoes, so stopping distance includes a longer application delay than hydraulic brakes.

At highway speed, total stopping distance is the sum of perception distance, reaction distance, brake lag distance, and actual braking distance; brake lag adds roughly a vehicle length at 55 mph.

Brake fade occurs when overheated drums and linings lose friction; it is managed by using engine braking and lower gears on long downgrades, not by riding the brakes.

On a long steep downgrade, select a gear low enough that the engine and retarder hold speed without continuous service brake use; apply brakes firmly and briefly to shed speed, then release.

Use the brakes in short, firm applications rather than light continuous pressure; continuous light application builds heat without enough cooling airflow.

The proper braking technique is to apply the brakes hard enough to slow the vehicle, then release fully to let the drums cool; repeated cycles are safer than a long drag.

Keep a following distance that accounts for the air brake lag and the heavier weight of the vehicle; a loaded air-braked truck needs substantially more distance than a passenger car.

If the vehicle starts to skid, release the brake pedal to allow the wheels to roll and regain traction, then reapply with less pressure.

If service brakes fail, downshift to a lower gear to use engine braking, apply the parking/spring brakes gradually, and look for an escape ramp or safe runoff.

Do not apply the spring brakes abruptly at high speed: they can lock the rear wheels and cause a skid or loss of control; apply them progressively.

If a low pressure warning sounds while driving, stop as soon as it is safe; continued driving risks automatic spring brake application at an unsafe moment.

If one circuit of a dual system fails, the other circuit still provides braking; expect longer stopping distance and a warning light, and stop to investigate.

If the trailer breaks away, the tractor protection valve and trailer emergency line cause the trailer spring brakes to apply, stopping the trailer.

If air pressure is lost completely, the spring brakes apply automatically; the vehicle can still be moved short distances only if the system has a functioning emergency release or the brakes are caged.

In an emergency stop, brake firmly and steer to maintain control; do not pump the brakes on a vehicle with anti-lock brakes, which modulate pressure automatically.

After any brake failure event, do not resume driving until the cause is found and corrected; the vehicle is out of service until repaired.

The parking brake is applied by spring pressure and released by air pressure; with the engine off and pressure drained, the spring brakes are applied and hold the vehicle.

Test the parking brake by applying it, releasing the service brakes, and confirming the vehicle does not roll on a slight grade; also check that the parking brake valve holds pressure.

Spring brake chambers have a mechanical release (caging) bolt used to release the brake for towing or when air is unavailable; caging does not repair the brake.

Never cage a spring brake and drive on it as a substitute for repair; caging is for moving a disabled vehicle to a safe location.

The parking brake valve (yellow diamond) should pop out and apply the spring brakes when air pressure drops below a set point, typically around 45 psi.

Inspect spring brake chambers for cracks, corrosion, and damaged caging bolts; a broken spring or chamber is an out-of-service condition.

When parking, set the parking brake, place the transmission in gear or park, and chock the wheels on a grade; do not rely on air pressure to hold the vehicle.

On a tractor-trailer, the trailer parking brake valve and the tractor protection valve work together; check that the trailer spring brakes apply when the trailer air supply is disconnected.

Drain all reservoirs daily and check for water and oil; a failed air dryer or worn compressor rings will send oil into the system and damage valves.

A compressor that builds pressure too slowly, runs continuously, or vents through the safety valve indicates a governor, unloader, or compressor fault and must be repaired.

Out-of-service conditions include: pressure buildup below the required rate, leaks exceeding the allowed drop, a low pressure warning that fails to activate before 60 psi, and a pushrod stroke beyond the adjustment limit.

A brake that does not release, a dragging brake, or a chamber with a broken spring or cracked housing is an out-of-service defect.

Automatic slack adjusters must be checked for correct adjustment; if the pushrod stroke exceeds the limit, the vehicle is out of service until the slack adjuster is repaired.

Hoses and fittings with cracks, chafing, or leaks are out-of-service; air leaks at fittings can cause pressure loss and delayed brake application.

The air dryer should be serviced at the manufacturer's interval; a saturated desiccant cartridge allows moisture into the system and causes freeze-ups in cold weather.

Keep a record of brake inspections and repairs; a vehicle with an unresolved brake defect must not be placed back in service.

Normal operating pressure is roughly 100-125 psi; the governor cut-out is near 120-125 psi and cut-in near 100 psi.

The low pressure warning must activate before pressure drops below 60 psi; below that, spring brakes may begin to apply.

The safety valve on the supply reservoir typically opens near 150 psi to protect the system from overpressure.

Brake chamber force equals air pressure times effective piston area; a Type 30 chamber at 100 psi produces about 3,000 lb of force.

Pushrod stroke is measured with the brakes applied and compared to the adjustment limit; excessive stroke means the slack adjuster needs adjustment or repair.

Pressure buildup requirement: 85 to 100 psi in about 45 seconds for a single vehicle; combination vehicles may take longer.

Static leak limit is 3 psi per minute with brakes released; applied leak limit is 4 psi per minute for a single vehicle.

Air brake lag adds distance before braking begins; at 55 mph, brake lag can add roughly one vehicle length to stopping distance.

The governor controls compressor output by unloading the compressor when reservoir pressure reaches cut-out and reloading at cut-in.

The foot valve (treadle valve) meters air to the brake chambers in proportion to pedal travel, giving graduated braking.

The relay valve speeds up brake application at the rear by using reservoir air locally rather than waiting for pressure from the foot valve.

The quick release valve exhausts air quickly when the brake is released, reducing release lag at the wheels.

The tractor protection valve protects the tractor air supply and controls trailer air; it closes if trailer pressure drops or the trailer breaks away.

The trailer spring brake valve applies the trailer spring brakes when the trailer air supply is lost or the emergency line is disconnected.

The double check valve allows the system to use the higher of two pressure sources, such as service and parking brake signals.

The alcohol evaporator or air dryer protects against moisture; the dryer is preferred because it removes water rather than adding alcohol to the system.

The low pressure warning device must be checked before each trip by draining pressure with the engine off and confirming the light and buzzer activate before 60 psi.

The spring brake test confirms that the brakes apply automatically when pressure is drained; with pressure above 100 psi, release the parking brake and then drain the system.

The static leak test allows no more than 3 psi drop in one minute with brakes released and the system fully charged.

The applied leak test allows no more than 4 psi drop in one minute with the brakes fully applied and held.

The pressure buildup test requires 85 to 100 psi in about 45 seconds with the engine at operating rpm for a single vehicle.

A pushrod stroke beyond the adjustment limit, a cracked chamber, a leaking hose, or a failed warning device is an out-of-service condition.

The parking brake must hold the vehicle on a grade; test it by applying the parking brake and confirming the vehicle does not roll.

The air dryer and reservoirs must be drained and inspected; water or oil in the system indicates a maintenance problem that must be corrected.

If pressure builds slowly, check the compressor, governor, unloader, and for leaks in the system; a slow buildup is a defect that must be repaired.

If the safety valve vents continuously, the governor or compressor unloader is likely stuck; stop and repair before driving.

If the low pressure warning fails to activate, the warning switch or wiring is faulty; the vehicle is out of service until repaired.

If one circuit of a dual system loses pressure, the other circuit still provides braking; expect a warning light and longer stopping distance, and stop to investigate.

If the spring brakes apply unexpectedly, check for air pressure loss, a faulty parking brake valve, or a leak in the spring brake circuit.

If a brake drags or does not release, check the quick release valve, relay valve, and chamber; a dragging brake overheats and can cause a fire.

If the compressor pumps oil into the system, the compressor rings or air dryer are failing; oil damages valves and causes brake failure.

If the trailer brakes do not apply, check the trailer emergency line, the tractor protection valve, and the trailer spring brake valve.

Brake lag means pressure takes time to reach the chambers; at 55 mph, lag can add about one vehicle length to stopping distance.

Use engine braking and lower gears on long downgrades; continuous light brake application causes heat buildup and fade.

Apply brakes firmly and briefly, then release to let drums cool; this is safer than a long continuous drag.

Do not pump the brakes on a vehicle with anti-lock brakes; the system modulates pressure automatically.

If the vehicle skids, release the brake pedal to let the wheels roll and regain traction, then reapply with less pressure.

Keep a following distance that accounts for brake lag and vehicle weight; a loaded air-braked truck needs more distance than a passenger car.

If service brakes fail, downshift, apply the parking/spring brakes gradually, and look for an escape ramp or safe runoff.

After any brake failure, do not resume driving until the cause is found and corrected; the vehicle is out of service until repaired.

Drain all reservoirs daily and check for water and oil; a failed air dryer or worn compressor rings will send oil into the system.

A compressor that builds pressure too slowly, runs continuously, or vents through the safety valve must be repaired.

Out-of-service conditions include: pressure buildup below the required rate, leaks exceeding the allowed drop, a low pressure warning that fails to activate before 60 psi, and a pushrod stroke beyond the adjustment limit.

A brake that does not release, a dragging brake, or a chamber with a broken spring or cracked housing is an out-of-service defect.

Automatic slack adjusters must be checked for correct adjustment; if the pushrod stroke exceeds the limit, the vehicle is out of service until repaired.

Hoses and fittings with cracks, chafing, or leaks are out-of-service; air leaks cause pressure loss and delayed brake application.

The air dryer should be serviced at the manufacturer's interval; a saturated desiccant cartridge allows moisture into the system and causes freeze-ups.

Keep a record of brake inspections and repairs; a vehicle with an unresolved brake defect must not be placed back in service.