NJ CDL Air Brakes Test cheat sheet

The compressor, driven by the engine, builds and maintains air pressure; a governor controls when the compressor loads (builds) and unloads (stops building), typically cutting out near 120-135 psi and cutting in around 100 psi.

The air dryer removes moisture and oil vapor downstream of the compressor; a faulty dryer sends water into the tanks and valves, causing freeze-ups and corrosion.

The reservoir (supply) tank stores compressed air; the wet tank sits closest to the compressor and collects the most moisture, so it is drained first and most often.

The foot valve (brake valve) is the driver's control: pressing the treadle meters air pressure from the reservoirs to the brake chambers in proportion to treadle travel.

The slack adjuster converts pushrod stroke into brake shoe rotation; its length multiplies pushrod force, so an out-of-adjustment or worn slack adjuster changes braking force at the wheel.

Brake chambers convert air pressure into mechanical force on the pushrod; a type 30 chamber has about 30 square inches of effective diaphragm area, so 100 psi produces roughly 3,000 pounds of pushrod force.

The relay valve speeds up application and release at the rear axle by supplying reservoir air directly to the brake chambers instead of routing it through the foot valve.

The spring (parking) brake chamber contains a heavy coil spring that applies the brakes when air pressure is released; air pressure must be present to compress the spring and release the brakes.

A dual air brake system has two separate circuits (primary and secondary), each with its own reservoir, so a failure in one circuit leaves the other able to stop the vehicle.

In the standard split, the primary circuit operates the rear brakes and the secondary circuit operates the front brakes; some vehicles split front/rear differently, so check the vehicle's labeling.

A one-way check valve between the two circuits prevents air from flowing from a failed circuit back into the good one, preserving pressure in the intact circuit.

The low pressure warning device must activate before or when pressure in the service brake system drops to 60 psi; a buzzer and/or light is required.

The spring brakes apply automatically when system pressure falls to roughly 20-45 psi (the tractor protection valve and spring brake priority valve drop out), so a slow leak can lead to an uncontrolled stop.

The tractor protection valve (trailer supply valve) closes the tractor-to-trailer air lines when tractor pressure drops to about 20-45 psi, keeping enough air for the tractor's own brakes and preventing trailer air loss.

A front-wheel limiting valve or automatic front brake proportioning reduces front brake pressure under light applications to reduce the chance of front-wheel lockup and loss of steering control.

The alcohol evaporator or air dryer injects alcohol or dries air to prevent moisture freezing in lines and valves; in cold weather, frozen moisture is a common cause of brake failure.

With the engine off and brakes released, listen for air leaks; a leak at a hose, fitting, chamber, or valve produces a hiss and must be fixed before driving.

Check the air compressor drive belt for cracks, glazing, and correct tension, and check the compressor for oil or coolant leaks.

Drain all reservoirs (wet tank first, then primary and secondary) and watch for water or oil; milky or oily discharge indicates compressor or dryer problems.

Check the air dryer for a missing, clogged, or damaged desiccant cartridge and for a heater that fails to operate in freezing weather.

Inspect brake chambers for dents, cracks, and a loose or broken clamp band; a chamber with a damaged diaphragm or spring is a defect that requires repair, not adjustment.

Check slack adjuster and pushrod travel against the vehicle's limits: with brakes applied, a typical limit is about 1 inch of stroke for a standard chamber, but always use the manufacturer's or inspector's specification.

Inspect hoses and tubing for chafing, cuts, bulges, and weather cracking, and check that lines are secured and not rubbing on the frame or suspension.

Verify that the low pressure warning light and buzzer work, and that the spring brakes apply and release properly, before the vehicle is moved.

Test the service brakes at low speed after the pre-trip inspection, and check that the vehicle does not pull to one side and that the brakes release fully.

Static leak test: with the engine off, full pressure built, and brakes released, observe the gauge; a loss of more than 3 psi in one minute (single vehicle) or 4 psi (combination) indicates a leak.

Applied leak test: with the engine off, press and hold the service brake treadle fully; a loss of more than 4 psi in one minute (single) or 6 psi (combination) indicates a leak.

Low pressure warning test: with the engine off, pump the brake pedal to reduce pressure; the warning light and buzzer must come on at or before 60 psi.

Spring brake test: continue pumping below the warning point; the parking brake valve should pop out and the spring brakes should apply at roughly 20-45 psi.

Service brake test at low speed (about 5 mph): apply the brakes firmly; the vehicle should stop smoothly without pulling, and the brake pedal should not feel spongy or bottom out.

Compressor build-up test: with the engine at operating rpm and pressure below cut-in, the gauge should rise from 85 to 100 psi within about 45 seconds (a common specification; check the vehicle's manual).

Governor cut-out and cut-in: the compressor should stop building at about 120-135 psi and resume at about 100 psi; a gauge that never reaches cut-out or cycles too often indicates a governor or compressor problem.

Air loss rate test for combination vehicles: charge the trailer, then shut the engine off and check that the tractor protection valve holds and that the trailer does not lose pressure faster than the allowed rate.

If any check fails, the vehicle is out of service until repaired; do not compensate for a leak or a slow build by driving with the warning ignored.

Build full air pressure before moving; check that the warning buzzer and light are off and the spring brakes are fully released.

Use the brakes in a way that keeps the vehicle straight: apply the pedal smoothly and progressively, and avoid sudden hard applications that can lock wheels on slick surfaces.

On long or steep downgrades, select a low gear before starting down and use engine braking; do not ride the service brakes continuously, which overheats drums and causes fade.

Use a stab-braking technique on long grades: apply the brakes firmly enough to slow the vehicle, then release fully to let the brakes cool, rather than holding light pressure.

Watch the air pressure gauge while driving; pressure should stay in the normal range (roughly 100-135 psi) and any continuous drop signals a leak or compressor problem.

Because air brakes have a lag, allow extra following distance; the brakes apply and release slightly after the pedal movement, and stopping distance is longer than with hydraulic brakes.

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

Before descending a grade, check that the brakes are adjusted and the air system is at full pressure, and know the escape route or pull-out if brakes fade.

If the low pressure warning sounds while driving, stop as soon as it is safe; continued driving drains the remaining air and the spring brakes will apply automatically.

If a brake chamber or line fails and air escapes rapidly, the spring brakes will apply when pressure drops to about 20-45 psi; steer to the shoulder and stop before that happens if possible.

If the service brakes fail, use the parking (spring) brake gradually; a full application at speed can lock the wheels and cause a skid, so apply it progressively.

If a wheel lockup or skid occurs, release the brake pedal to regain steering control, then reapply with less pressure; steer in the direction you want to go.

If the compressor fails, the remaining reservoir air is limited; stop promptly, because once pressure falls the spring brakes apply and cannot be released without air.

In a runaway situation on a downgrade, use the escape ramp or a safe off-road area; do not attempt to downshift into a gear that would overspeed the engine.

After any emergency stop caused by an air system failure, do not continue until the cause is found and repaired; a system that lost pressure once will do so again.

If the trailer breaks away, the trailer's emergency relay valve applies the trailer spring brakes automatically when the supply line separates.

Water in the reservoirs is the most common cause of air brake trouble; drain the wet tank daily and the other tanks regularly, and repair a failed air dryer.

Oil in the tanks or at the drain indicates a worn compressor or bad compressor rings; oil damages rubber seals and valves and must be corrected, not just drained.

A compressor that builds pressure slowly can be caused by a worn compressor, a slipping or broken drive belt, a clogged air dryer, or a leak; isolate the cause before replacing parts.

A governor that fails to cut out leaves the compressor building until the safety valve releases at about 150 psi; a governor that cuts in too low causes frequent cycling.

A safety valve (pop-off valve) that releases air continuously indicates a stuck governor or excessive pressure; it is a warning, not a normal vent.

Brakes that drag or do not release can be caused by a stuck relay valve, a kinked or plugged line, a broken return spring, or a maladjusted slack adjuster.

Automatic slack adjusters should not be manually adjusted to compensate for excessive pushrod travel; if travel exceeds the limit, the adjuster or chamber needs repair.

Check the air dryer cartridge and the compressor discharge line for carbon buildup; a restricted discharge line can cause slow build-up and overheating.

Any repair to the air brake system must restore it to the manufacturer's specification; after repair, repeat the leak, warning, and build-up tests before returning the vehicle to service.