KS CDL Air Brakes cheat sheet

Compressor: engine-driven pump that builds and maintains air pressure; typically governed to cut out near 120-125 psi and cut in near 85-100 psi.

Governor: senses reservoir pressure and signals the compressor to load (cut-in) or unload (cut-out), preventing over-pressurization.

Air dryer: removes moisture and oil vapor downstream of the compressor; purge valve cycles each unload cycle, and the desiccant cartridge has a finite service life.

Reservoirs: store compressed air; the wet tank receives compressor air first, while dry tanks supply the brake circuits.

Safety valve: spring-loaded relief device on the wet tank or supply reservoir that vents air if pressure exceeds its setting, typically about 150 psi.

Foot valve (treadle valve): driver-operated demand valve that meters air pressure to the brake circuits in proportion to pedal travel.

Relay valve: mounted near the brake chambers; uses a small control signal to deliver a large volume of air quickly, reducing lag time.

Slack adjusters and brake chambers: the chamber converts air pressure into mechanical force; the slack adjuster converts rod stroke into brake-shoe rotation.

Low-pressure warning switch and buzzer/light: activates when system pressure drops below a threshold, commonly around 60 psi.

Dual systems split service brakes into two independent circuits (typically front axle and rear axle) so a single failure leaves partial braking capability.

Check valves isolate the two circuits so that a leak or failure in one circuit does not drain pressure from the other.

Low pressure warning must activate before pressure falls below 60 psi; the warning is both audible and visual.

Spring brakes (emergency/parking brakes) are held applied by heavy springs and released by air pressure; loss of air causes automatic application.

The spring brake parking valve (yellow diamond knob) releases air to apply the springs and supplies air to release them.

A tractor protection valve closes the trailer supply line when pressure drops, preventing loss of tractor air through a broken trailer connection.

The trailer supply line (red coupler) and service line (blue coupler) are color-coded; the red line charges the trailer, the blue line carries brake application signals.

The park control must not be used for normal stopping; it is for parking and emergency application of spring brakes.

Antilock braking systems (ABS) modulate pressure at individual wheels; ABS malfunction does not remove normal service braking.

Static leak test: with engine off and full pressure, release the parking brake and hold the service brake; pressure loss must not exceed 3 psi in one minute for a single vehicle.

For a tractor-trailer combination, the same one-minute service-brake leak test allows no more than 4 psi total loss.

Spring brake leak test: with the parking brake applied and engine off, loss must not exceed 3 psi in one minute (single) or 4 psi (combination).

Low pressure warning test: with engine off, pump the brake pedal to reduce pressure; the warning must sound and light before pressure drops below 60 psi.

Spring brake application test: continue pumping until the parking brake knob pops out; this must occur between 20 and 45 psi.

Pressure build-up test: with the system fully charged and engine at operating rpm, pressure must rise from 85 to 100 psi within 45 seconds.

Compressor governor cut-out is typically near 120-125 psi and cut-in near 85-100 psi; verify both during the buildup check.

Check the air dryer for moisture discharge, drain all reservoirs of water and oil, and confirm drain cocks close properly.

Inspect slack adjuster free play, brake chamber pushrod travel, hoses, and glad hands for cracks, chafing, and leaks.

Build full air pressure before moving; do not drive until the low pressure warning has stopped and pressure is at normal operating range.

Perform a brake application check at low speed after starting: apply brakes firmly and confirm the vehicle pulls evenly and stops.

Use progressive, steady pedal pressure; air brakes are not applied by pedal force alone but by metered air pressure, so light steady pressure is effective.

Allow for longer stopping distances than hydraulic brakes; air brake lag time adds distance at speed.

Use engine braking and downshifting on long downgrades; continuous light service brake application can overheat drums and fade brakes.

On a long descent, select a gear that holds a safe speed without riding the brakes; apply brakes in short firm applications if speed builds.

Do not fan or pump the pedal repeatedly on a downgrade; each application consumes air and can deplete the system.

Watch the air pressure gauge continuously while driving; pressure should remain in the normal range and never fall below the warning threshold.

Before parking, apply the parking brake and confirm the spring brakes are set; use wheel chocks on grades.

If the low pressure warning activates while driving, stop as soon as it is safe; continued driving risks automatic spring brake application.

If service brakes fail, downshift progressively, use engine braking, and apply the parking brake gradually to avoid locking the wheels.

If a spring brake applies unexpectedly at speed, keep steering control and apply the parking brake control to modulate if possible; do not release it while the vehicle is still moving fast.

If an air line or glad hand fails, the tractor protection valve should close and preserve tractor air; stop and repair before continuing.

Use runaway truck ramps or escape ramps if brakes cannot control speed on a grade; steer for the ramp rather than continuing downhill.

If brakes overheat and fade, stop and let them cool; do not continue downhill with reduced braking.

In a brake emergency, warn other traffic with hazard lights and horn while bringing the vehicle to a controlled stop.

After any emergency brake application, inspect the system for leaks and confirm pressure builds before resuming driving.

If a wheel lock or skid occurs, release brake pressure momentarily to regain steering control, then reapply with less force.

Drain air tanks daily; moisture and oil accumulation can damage valves and cause freeze-ups in cold weather.

Alcohol evaporators introduce alcohol vapor into the air system to prevent moisture from freezing in lines and valves during cold weather.

The alcohol evaporator reservoir must be kept filled with the specified alcohol; do not substitute other fluids.

Air dryer desiccant cartridges require periodic replacement; a saturated cartridge passes moisture downstream and causes freeze-ups.

Excessive compressor oil carryover indicates worn compressor rings or seals and can foul valves and the air dryer.

A compressor that runs continuously without cutting out may indicate a faulty governor or a system leak; check both.

Slow pressure buildup can result from a worn compressor, a leaking line, or a restricted intake; isolate by checking for leaks first.

Slack adjuster pushrod travel beyond the adjustment limit requires immediate service; automatic slack adjusters must be checked and set correctly.

Brake chamber diaphragm rupture causes a continuous air leak and loss of braking on that wheel; replace the diaphragm rather than plugging the line.