Pennsylvania CDL Air Brakes cheat sheet

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

The air compressor intake draws air through the engine air filter on most systems, so a clogged engine filter can slow pressure buildup and must be checked during inspection.

An air dryer removes moisture and oil vapor after the compressor; a failed dryer or saturated desiccant lets water reach the tanks and valves, where it can freeze in cold weather.

The supply (wet) tank is the first tank after the compressor and dryer; it collects condensed water and delivers air to the other tanks through a one-way check valve.

One-way check valves between tanks keep air from flowing backward, so a leak in one part of the system does not drain the entire air supply.

The foot valve (brake valve) is the treadle-operated valve that meters air from the tanks to the brake chambers in proportion to pedal travel, giving the driver control of braking force.

A relay valve on long-wheelbase or multi-axle vehicles speeds application and release by sending tank air directly to the brake chambers instead of routing it all through the foot valve.

Slack adjusters transmit pushrod force to the brake shoes; with the brakes released and pushed by hand, the pushrod should move no more than about 1 inch on a standard chamber (check the manufacturer's specification).

A safety valve on the supply tank protects the system by releasing air if pressure rises above its setting, typically around 150 psi.

A dual air brake system has two separate circuits, usually a primary circuit for the rear brakes and a secondary circuit for the front brakes, each with its own tank and check valve.

The two circuits share the same compressor and supply tank but are isolated by one-way check valves, so a failure in one circuit does not automatically drain the other.

If one circuit loses pressure, the other continues to supply braking; the driver should still see a pressure drop on the gauge for the failed circuit and must stop to check the system.

In a dual system the low air pressure warning must activate before pressure in either circuit drops below 60 psi, and the spring brakes must apply when pressure falls into the 20-45 psi range.

The primary and secondary gauges should be checked separately during inspection; a single gauge may show only one circuit, so know which gauge reads which circuit on your vehicle.

The foot valve in a dual system has two separate supply and delivery sections, one per circuit, so a seal failure inside the valve affects only one circuit.

A dual system requires that the compressor build pressure from 85 to 100 psi within 45 seconds at fast idle with the engine running and the system fully charged.

The tractor protection valve on combination vehicles closes and prevents air loss from the tractor if the trailer breaks away or a trailer line is severed.

With the engine running at fast idle, the system should build from 85 to 100 psi within 45 seconds; longer buildup indicates a compressor, governor, or leak problem.

With the engine off and the system fully charged, the governor cut-out pressure should be at least 100 psi and the cut-in at least 85 psi; record both readings from the gauge.

With the engine off, release the parking brake, apply and hold the foot brake for one minute, and check that pressure loss does not exceed 3 psi for a single vehicle or 4 psi for a combination vehicle.

With the engine off and the parking brake released, hold the brake pedal and listen for leaks; a leak at a chamber, hose, or fitting must be repaired before the vehicle is operated.

Check that the low air pressure warning activates before pressure falls below 60 psi by pumping the brake with the engine off and watching for the light and buzzer.

With the engine off and pressure below 60 psi, continue pumping and confirm the spring brakes apply; the parking brake valve should pop out between roughly 20 and 45 psi.

Test the spring brakes by pulling the parking brake valve with the engine off and transmission in a low gear, then attempt to pull forward; the vehicle must not move.

Check the slack adjusters by pulling each pushrod by hand with the brakes released; more than about 1 inch of free play on a standard chamber indicates adjustment is needed.

Check all hoses and fittings for cracks, wear, and air leaks, and drain moisture from the supply and service tanks at the end of each day of operation.

Static leak test: with a fully charged system and the engine off, a single vehicle should lose no more than 3 psi in one minute and a combination vehicle no more than 4 psi.

Applied leak test: with the engine off and the brakes held, loss should not exceed 3 psi per minute for a single vehicle or 4 psi per minute for a combination.

A steady hiss with the brakes released points to a supply-side leak, such as a tank, hose, or fitting; a hiss only with the brakes applied points to a delivery-side leak, such as a chamber or valve.

A compressor that runs continuously, or a governor that never cuts out, usually indicates an air leak or a faulty governor rather than a normal high-demand condition.

Water in the tanks, visible as moisture when draining, means the air dryer is not working; excess water can freeze in lines and valves and cause brake failure in cold weather.

A pressure gauge that falls rapidly when the engine is shut off, or that will not reach cut-out pressure, signals a serious leak that must be repaired before driving.

The alcohol evaporator or air dryer protects against freeze-up, but it does not substitute for draining tanks and repairing leaks.

The low air pressure warning must be a visible and audible signal, typically a red light and a buzzer, that activates before pressure in either circuit falls below 60 psi.

On vehicles with a dual system, the warning must work for both circuits; test each circuit separately if the vehicle has separate gauges.

A warning that fails to activate, or that activates only at very low pressure, is a defect that places the vehicle out of service until repaired.

The warning is the driver's cue to stop safely and investigate; continued driving with the warning on leads to spring brake application at roughly 20-45 psi.

Some vehicles also have a pressure gauge with a red danger zone; the warning device is the primary alert, and the gauge is a secondary indicator.

Test the warning by pumping the brake pedal with the engine off and the parking brake released, watching for the light and listening for the buzzer before pressure reaches 60 psi.

Spring brakes are held applied by a heavy spring and released by air pressure; when air pressure is lost, the spring applies the brakes automatically.

Spring brakes apply automatically when system pressure drops to roughly 20-45 psi, which is why the low air pressure warning must come on well before that point.

The parking brake valve (yellow diamond knob) releases air from the spring brake chambers to apply the brakes; pushing the knob in recharges the chambers and releases them.

Never attempt to release a spring brake by caging it and driving; caging is for moving a disabled vehicle only, and caged brakes provide no emergency braking.

The spring brake chambers on a combination vehicle are on the tractor and trailer; a breakaway or severed line causes the tractor protection valve to close and the trailer spring brakes to apply.

In an emergency, the spring brakes can be applied gradually by pulling the parking brake valve; this is a controlled emergency stop, not a substitute for normal braking.

Before driving, check that the spring brakes hold the vehicle by applying the parking brake with the transmission in a low gear and attempting to move forward.

On a vehicle with a modulated spring brake valve, the parking brake knob can be used for emergency braking; on vehicles without it, the knob applies the brakes fully and abruptly.

The parking brake valve is the yellow diamond knob; pulling it out applies the spring brakes, and pushing it in releases them after the system recharges.

The parking brake must hold the vehicle on a grade; test it by applying the brake, placing the transmission in a low gear, and attempting to move forward.

Never leave the vehicle with the engine running and the parking brake released; a vehicle can roll if the spring brakes are not applied.

On a grade, set the parking brake, place the transmission in the lowest gear (or park for an automatic), and use wheel chocks where required.

The parking brake should not be used as a substitute for service brakes during normal driving; use it only for parking or emergency stops.

Before releasing the parking brake, confirm that system pressure is at normal operating level, since the spring brakes will not release fully without sufficient air.

On combination vehicles, the trailer parking brake (if equipped) and the tractor parking brake should both be set when parking on a grade.

Air brakes require a longer stopping distance than hydraulic brakes; maintain a following distance of at least one second per 10 feet of vehicle length plus one second, and more in poor conditions.

Use the brakes in a way that keeps the system charged: avoid riding the brakes on long downgrades, and use engine braking and a lower gear to control speed.

Brake fade from overheated drums reduces stopping power; if brakes fade, use a lower gear and a safe pull-off to let the brakes cool, and do not continue at speed.

Air brake systems have a lag between pedal movement and brake application; allow for this lag when judging stopping distance.

On wet roads, air brakes can be slow to release and may pull to one side; test the brakes gently after driving through water.

The parking brake valve should be used for parking only; during normal driving, the foot valve controls the service brakes.

Before descending a long grade, check that system pressure is at normal operating level and select a gear that will hold the vehicle without continuous braking.

If the low air pressure warning activates while driving, stop as soon as it is safe to do so and do not continue until the system is repaired.