KY CDL Air Brakes cheat sheet

The compressor, driven by the engine, builds and maintains air pressure; a governor controls when the compressor loads (pumps) and unloads (stops pumping), typically cutting out near 120-125 psi and cutting in near 100 psi.

The air compressor intake draws filtered air; a faulty or restricted intake, loose belt, or carbon buildup reduces the compressor's ability to build pressure and is a common cause of slow pressure buildup.

The air dryer removes moisture and oil vapor from compressed air before it reaches the reservoirs; a saturated or failed dryer cartridge lets water into the system and can cause brake valve freeze-up in cold weather.

Reservoirs (tanks) store compressed air; a supply (wet) tank receives air first and a primary and secondary tank feed the two brake circuits through a one-way check valve that prevents backflow.

The foot valve (brake valve) is the treadle-operated control that meters air pressure to the brake chambers in proportion to how far the pedal is pushed; releasing the pedal exhausts air and releases the brakes.

The brake chamber converts air pressure into mechanical force; a spring brake chamber contains a strong spring that applies the brakes when air pressure is lost, and a service chamber applies brakes only while air is delivered.

A slack adjuster converts the pushrod stroke into rotation of the brake camshaft (S-cam brakes); automatic slack adjusters maintain clearance automatically, but excessive pushrod stroke still indicates a needed repair.

A low-pressure warning device must activate before pressure drops below 60 psi; the warning is both a visual signal and an audible buzzer or alarm that alerts the driver to a pressure loss.

A dual air brake system has two separate circuits (primary and secondary) fed from separate reservoirs, so a failure or leak in one circuit does not deplete the other; each circuit must hold pressure independently.

In a typical dual system the primary circuit operates the rear brakes and the secondary circuit operates the front brakes; on a combination vehicle the tractor protection valve controls air to the trailer.

The tractor protection valve closes automatically when tractor air pressure falls to about 20-45 psi, preventing loss of tractor air through a broken trailer line and applying the trailer spring brakes.

A one-way check valve at each reservoir inlet keeps air from flowing back out of a tank if the supply line or compressor fails, preserving whatever pressure remains in that circuit.

The spring (parking) brakes are held applied by mechanical spring force and released only by air pressure; a loss of air pressure therefore results in automatic brake application rather than loss of braking.

The parking brake control (yellow diamond knob) releases air to apply the spring brakes; pulling it out applies the parking brakes, and it should not be used as a substitute for the service brakes during normal stopping.

The safety valve on the supply tank is a pressure relief device set to open around 150 psi; it protects the system if the governor fails and pressure rises above safe limits.

A front brake limiting valve or automatic front brake proportioning reduces front brake force in some older systems; modern systems require front brakes to be fully functional, so know the configuration of the vehicle you drive.

The low air pressure warning and the spring brakes together form the fail-safe design: warning gives the driver time to stop safely before spring brakes apply automatically at very low pressure.

Check the air compressor drive belt for cracks, glazing, and correct tension, and check for oil or coolant leaks at the compressor; a slipping belt prevents the system from reaching governed pressure.

Drain all air reservoirs daily (or as required) to remove moisture and oil; open the drain cocks fully until no water comes out, then close them, and note that a failed air dryer increases water accumulation.

Check the air dryer for a clogged or saturated desiccant cartridge and confirm the purge valve operates; moisture carryover can cause brake valves to stick or freeze.

Inspect all air lines and hoses for chafing, cracks, and leaks at fittings; a soapy water solution on fittings and hoses reveals bubbles at leaking connections.

Check the brake chambers for dents, cracks, and a loose or misaligned clamp ring; a damaged chamber or a pushrod that is bent or misaligned must be repaired before driving.

Measure pushrod stroke against the vehicle's specification with the brakes fully applied and the system at normal pressure; stroke beyond the limit means the brakes are out of adjustment and the vehicle is out of service.

Inspect the slack adjusters for a missing or broken locking pawl or clevis pin, and check that the adjuster is not frozen; a frozen automatic slack adjuster will not maintain correct stroke.

Check the low-pressure warning device by building pressure and then reducing it; confirm the light and buzzer activate before pressure falls below 60 psi.

Check the brake linings and drums through the inspection holes for thickness, oil contamination, and cracks; contaminated or worn linings reduce stopping ability and cannot be corrected by adjustment.

Static leak test: with the engine off, the system fully charged, and the brakes released, observe pressure for one minute; a loss greater than 2 psi in one minute (or the manufacturer's limit) indicates a leak.

Applied leak test: with the engine off and the brakes fully applied, hold for one minute; a loss greater than 3 psi in one minute (or the manufacturer's limit) indicates a leak.

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

Governor check: build pressure until the compressor unloads; cut-out typically occurs around 120-125 psi and cut-in around 100 psi, and both should be within the manufacturer's specification.

Build-up rate check: with the system at low pressure and the engine at operating rpm, pressure should rise from about 85 to 100 psi within 45 seconds (check the specific vehicle's specification).

Spring brake test: with the system charged, shut off the engine and fan off air pressure; the spring brakes should apply automatically when pressure drops into the 20-45 psi range.

Tractor protection valve test: with the trailer charged and the engine off, close the trailer air supply valve; the trailer spring brakes should apply and the tractor protection valve should prevent air loss.

Compressor check: with the engine running at fast idle and the system at governed pressure, the compressor should maintain pressure and the governor should cycle normally; a system that cannot reach cut-out has a compressor or leak problem.

Always chock the wheels and use wheel chocks before performing tests that may release the parking brake or apply spring brakes on an incline.

Air brakes have a lag between pedal movement and brake application; apply the pedal smoothly and allow time for pressure to build in the chambers before expecting full braking force.

Brake application force is proportional to pedal travel, not pedal effort; pushing harder than needed does not increase braking beyond the available air pressure and can cause wheel lockup.

Use controlled, progressive braking and avoid fanning or pumping the pedal, which wastes air and can reduce reservoir pressure below safe levels on a long downgrade.

Air brake systems are designed so that a loss of pressure applies the spring brakes; do not rely on holding the pedal down to keep the vehicle stationary, because pressure bleeds off over time.

At highway speed, maintain a following distance that accounts for the longer stopping distance of a heavy vehicle; the air brake lag adds distance beyond the reaction and braking distance.

When stopping on a grade, set the parking brake and use wheel chocks; the spring brakes are the parking brake, and the service brakes should not be used to hold a parked vehicle.

Watch the air pressure gauge while driving; if pressure falls below the governed cut-in and does not recover, stop safely and investigate before continuing.

In cold weather, moisture in the system can freeze valves and lines; drain tanks, maintain the air dryer, and allow extra time for the system to build pressure before driving.

Do not ride the brakes or use the trailer hand valve for normal stopping; the hand valve is for testing trailer brakes and can cause a jackknife if used while moving.

Before descending a long or steep grade, check the air pressure gauge, select a gear low enough that engine braking holds the speed, and do not rely on the service brakes to control speed.

Use the snub braking technique: apply the brakes firmly to reduce speed by about 5 mph, then release and let the engine brake hold; repeat as needed rather than holding continuous pressure.

If air pressure drops below the warning threshold, stop as soon as it is safe, set the parking brake, and do not continue until the problem is corrected; spring brakes will apply at very low pressure.

If the service brakes fail, downshift progressively to use engine braking, use the emergency/parking brake control gradually, and look for an escape ramp or a safe runoff area.

An escape ramp is designed to stop a runaway vehicle; steer into it rather than attempting to continue down the grade, and follow posted instructions for the ramp.

If a trailer breaks away or an air line separates, the tractor protection valve and trailer spring brakes should apply automatically; do not attempt to reconnect lines while the vehicle is moving.

In a brake emergency, avoid locking the wheels; locked wheels reduce steering control and increase stopping distance, so apply brakes firmly but progressively.

After any emergency stop or pressure loss, inspect the system for leaks, damaged lines, and proper pressure recovery before resuming the trip.