Ohio CDL Air Brakes Test cheat sheet

Compressor: engine-driven pump that builds and maintains air pressure; governor unloads it at cut-out pressure (about 125 psi) and reloads at cut-in (about 100 psi).

Air compressor types: belt-driven or gear-driven; most CDL vehicles use a reciprocating piston compressor with a governor-controlled unloading mechanism.

Reservoirs: storage tanks that hold compressed air; a check valve at the compressor discharge and at each tank inlet prevents backflow and pressure loss if the compressor fails.

Safety valve: spring-loaded valve on the supply (wet) tank that vents at about 150 psi to prevent tank rupture if the governor fails.

Drain valves: located at the bottom of each reservoir to remove moisture and oil; the supply tank collects the most water and must be drained daily.

Foot valve (brake valve): treadle-operated valve that meters air from the reservoirs to the brake chambers in proportion to pedal travel.

Brake chambers: convert air pressure into mechanical force; the pushrod stroke moves the slack adjuster, which rotates the S-cam to apply the brakes.

Slack adjusters: connect the pushrod to the brake camshaft; manual slack adjusters require periodic adjustment, while automatic slack adjusters self-adjust during normal brake applications.

Low-pressure warning device: buzzer and/or light that activates when system pressure falls below about 60 psi, warning of insufficient pressure to stop safely.

Dual air brake systems use two separate circuits (primary and secondary) so a failure in one circuit still leaves braking capability in the other.

Primary circuit typically operates the rear (drive) axle brakes; secondary circuit typically operates the front (steering) axle brakes, with some vehicles using a split front/rear arrangement.

Each circuit has its own reservoir, check valve, and low-pressure indicator; the circuits share the foot valve and the compressor supply.

The foot valve is a dual-circuit treadle valve that meters air to both circuits simultaneously; loss of one circuit still allows the other to function.

The tractor protection valve (TPV) is a key safety device: it closes automatically when tractor air pressure drops below about 20-45 psi, preventing air loss from the tractor and protecting the tractor brakes.

The TPV also has a manual control (trailer air supply valve) in the cab that the driver uses to charge or release the trailer air lines.

The trailer supply valve (red button) and trailer parking valve (yellow button) are push-pull controls; pulling the red button out closes the TPV and applies trailer spring brakes.

A quick-release valve on the trailer speeds release of trailer brakes when the foot valve is released, improving stopping response.

Dual-circuit systems require that both circuits be charged before the vehicle can be driven; a low-pressure condition in either circuit triggers the warning device.

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 more than 4 psi in one minute for a combination vehicle (static leak test).

Low-pressure warning test: with the engine off and parking brakes released, pump the brake pedal to reduce pressure; the low-pressure warning light and buzzer must activate before pressure drops below about 60 psi.

With the engine running, the compressor should build pressure from 85 to 100 psi within 45 seconds (compressor build-up test) for a typical vehicle.

The governor cut-out pressure should be about 125 psi and cut-in about 100 psi; if the compressor runs continuously or never unloads, the governor or compressor may be faulty.

The TPV check: with the system charged and the engine off, close the TPV (red button out), then open the trailer air supply valve; the TPV should prevent air from escaping from the tractor.

The TPV should also close automatically when tractor pressure drops below about 20-45 psi; this is verified by watching the trailer spring brakes apply during a leak-down test.

Inspect the air lines and glad hands for cracks, leaks, and secure connections; make sure the trailer supply and service lines are not crossed.

Check that the low-pressure warning device works in both circuits; a failed warning device means the vehicle is out of service until repaired.

Walk-around inspection: check slack adjuster travel, brake chamber pushrod stroke, brake lining thickness, and drain all reservoirs before driving.

Static leak test: with the engine off and system fully charged, observe pressure drop over one minute; allowable loss is 3 psi for a single vehicle and 4 psi for a combination vehicle.

Applied leakage test: with the engine off, fully apply the brake pedal (or service brakes) and hold; the pressure should not drop more than 3 psi in one minute for a single vehicle or 4 psi for a combination.

Compressor build-up test: with the engine at operating rpm, the system should build from 85 to 100 psi within 45 seconds; slower build-up indicates a weak compressor or leak.

Low-pressure warning test: pump the brake pedal with the engine off until the warning buzzer/light activates; it must come on before pressure falls below about 60 psi.

TPV test (manual): charge the system, close the TPV (pull red button), and open the trailer air supply valve; air should not escape from the tractor's trailer lines.

TPV test (automatic): with the trailer connected and charged, release the parking brake and observe that the trailer spring brakes release; then reduce tractor pressure and verify the TPV closes and the trailer brakes apply.

Governor test: run the engine until the compressor unloads (cut-out about 125 psi), then observe that it reloads at cut-in (about 100 psi); a governor that fails to unload causes continuous compressor operation.

If any test fails (leak rate too high, warning fails, TPV fails), the vehicle must be placed out of service until repairs are made.

Always chock the wheels and set the parking brake before performing tests; never crawl under a vehicle that is not properly secured.

Normal stop: release the accelerator, apply the brake pedal smoothly and progressively; air brakes have a lag time, so anticipate stopping distance and apply earlier than with hydraulic brakes.

Brake fade: repeated heavy braking overheats the drums and reduces stopping power; use engine braking (downshifting) on long downgrades to control speed and avoid overheating the brakes.

On long downgrades, select a low gear before descending and use the brakes intermittently rather than riding them continuously.

Emergency stop: apply the brake pedal firmly and hold it; do not pump the brakes, as this wastes air and reduces braking effectiveness.

If the brakes fail, downshift to slow the vehicle, use the emergency/parking brake gradually, and look for an escape route (runaway ramp, uphill grade, or clear area).

Stopping distance increases with speed and load; maintain a following distance of at least one second per 10 feet of vehicle length, plus one second for speeds above 40 mph.

Brake lag: air brakes take about 0.3-0.5 seconds to apply after the pedal is pressed; this adds to total stopping distance and must be accounted for.

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

Release the parking brake only after the system is fully charged and the low-pressure warning has gone off; never drive with the warning active.

Spring brakes are the parking and emergency brakes: a heavy coil spring applies the brakes when air pressure is released, and air pressure is required to release (cage) the spring.

The spring brake chamber has two sections: a service chamber (air-applied) and a spring chamber (spring-applied); a divider separates them.

The parking brake control (yellow button) releases air from the spring brake chambers to apply the brakes; pushing the button in recharges the chambers to release the brakes.

The trailer air supply valve (red button) controls the tractor protection valve and the trailer spring brakes; pulling it out applies the trailer parking brakes.

If air pressure drops below about 20-45 psi, the spring brakes apply automatically as a fail-safe, stopping the vehicle.

To release a spring brake manually (caging), insert the caging bolt and tighten it to compress the spring; this is a temporary measure for towing or moving a disabled vehicle.

Never cage a spring brake and then drive normally; the brake will not function and the vehicle is unsafe.

The emergency brake system uses the spring brakes as a backup when service brakes fail; a loss of air pressure applies the spring brakes automatically.

Always set the parking brake and chock the wheels before leaving the vehicle; spring brakes can hold on grades but wheel chocks provide additional security.

ABS prevents wheel lock-up during braking by modulating air pressure to individual wheels or axles, improving steering control and reducing stopping distance on slippery surfaces.

ABS does not replace normal braking; the driver should apply the brakes normally and let ABS modulate pressure when wheel slip is detected.

The ABS warning light (amber) should come on when the ignition is turned on and go off after a few seconds; if it stays on, the ABS is malfunctioning and should be repaired.

If the ABS warning light stays on, the vehicle still has normal air brakes, but ABS is disabled; drive with caution and have the system checked.

Common air brake malfunctions: low pressure (warning at about 60 psi), compressor failure, governor failure, leaky lines or chambers, and a stuck TPV.

A failed compressor or governor can cause pressure to drop or fail to build; if pressure will not build, do not drive until repaired.

A leaking brake chamber or line causes a pressure drop and may prevent the spring brakes from releasing; inspect for audible leaks and worn seals.

If the low-pressure warning activates while driving, pull over safely, set the parking brake, and investigate; do not continue driving with low air pressure.

ABS on trailers: the trailer ABS light should be checked during the walk-around; a trailer ABS malfunction does not affect tractor brakes but should be reported and repaired.