NY CDL Air Brakes cheat sheet

The compressor, driven by the engine, builds and maintains air pressure; it must build pressure fast enough to keep up with use and normally builds 85-100 psi within a set time at fast idle.

The air compressor governor controls when the compressor pumps: it cuts in at about 100 psi and cuts out at about 125 psi, so the system cycles between those pressures.

The air storage tanks hold compressed air; a supply tank feeds the system and a separate dry tank or wet tank arrangement traps moisture and oil before air reaches the brakes on many vehicles.

The foot valve (treadle valve) is the brake pedal: it meters air pressure from the tanks to the brake chambers in proportion to how far the pedal is pressed.

The brake chamber converts air pressure into mechanical force; the pushrod moves a slack adjuster, which twists the brake camshaft (S-cam) to press the shoes against the drum.

The slack adjuster is the lever between pushrod and camshaft; its length and angle set the mechanical advantage and it must be adjusted so the pushrod stroke stays within the manufacturer's limit.

The air dryer removes moisture and oil from compressed air before it reaches the tanks, and its purge cycle and heater protect the system from freeze-up in cold weather.

A safety valve on the air tank opens at about 150 psi to protect the system from overpressure if the governor fails.

The low pressure warning switch and the compressor's check valve prevent backflow: the check valve keeps air from leaking back through the compressor when it stops pumping.

A dual air brake system has two separate circuits (primary and secondary) fed from the same compressor and tanks, so a failure in one circuit leaves the other with working brakes.

In the standard split, the primary circuit operates the rear brakes and the secondary operates the front brakes; on some vehicles the split is diagonal or by axle, so know the vehicle's configuration.

Each circuit has its own reservoir and check valve, so a leak in one circuit does not drain the other; the check valves prevent air from flowing back from one circuit to the other.

The dual foot valve has two separate sections that deliver air to the two circuits from a single pedal, so pressing the pedal applies both circuits simultaneously.

A pressure gauge with two needles (or two gauges) shows the pressure in each circuit; the needles should read within a normal range and should not drop when the brakes are released.

If one circuit loses pressure, the other circuit still stops the vehicle, but stopping distance increases and the low pressure warning may come on for the failed circuit.

The system must be protected by a pressure differential valve or similar device that warns the driver when pressure between the two circuits differs significantly.

A spring brake (parking brake) is held applied by a heavy spring and released by air pressure; a loss of air pressure in the spring brake circuit causes the brakes to apply automatically.

When draining tanks, the primary and secondary reservoirs must be drained separately, and both must be recharged before the vehicle is driven.

Before driving, check that the air pressure builds from 85 to 100 psi within the time specified by the manufacturer (typically about 45 seconds at fast idle for a typical system).

Perform the static leak test: with the engine off and the system fully charged, apply and hold the brakes and watch the gauge; pressure should not drop more than 3 psi in one minute for a single vehicle (5 psi for a combination).

Check the low pressure warning: with the engine off, pump the brakes to reduce pressure; the warning light and buzzer must activate by 60 psi or the manufacturer's specified pressure.

Check that the spring brakes apply automatically: continue releasing air until the parking brake knob pops out, which must occur between 20 and 45 psi on most systems.

With the engine running and the system charged, release the parking brake and check that the governor cuts out and the compressor stops pumping at the cut-out pressure (about 125 psi).

Check the slack adjusters: with the brakes released, mark the pushrod and apply the brakes fully; the pushrod stroke must not exceed the manufacturer's limit (commonly 1 inch for a type 30 chamber at 90 psi).

Inspect the brake chambers, hoses, and lines for cracks, chafing, and leaks; soapy water applied to fittings will bubble at a leak.

Test the parking brake by applying it and trying to move the vehicle with light throttle; the vehicle should not move.

Drain all air tanks of moisture and contamination at the end of the day, and check the air dryer for proper operation.

The parking brake control (yellow diamond knob) is pushed in to release and pulled out to apply; it controls the spring brakes, which apply automatically when air pressure is lost.

The trailer air supply control (red octagon knob) supplies air to the trailer; it pops out automatically when pressure drops to about 45 psi, applying the trailer spring brakes.

The foot valve applies the service brakes progressively: more pedal travel delivers more air pressure, so the driver can modulate braking force.

The engine retarder (if equipped) must be turned off on slippery surfaces, because it can cause the drive wheels to lose traction; it does not replace the service brakes.

The brake pedal should not be pumped on a vehicle with air brakes; hold steady pressure, because pumping releases air and can reduce braking force.

The parking brake should not be used for emergency stopping at highway speed; use the foot brake and downshift as needed.

The air compressor governor controls the compressor's cut-in and cut-out; the driver can hear the compressor cycle as it builds pressure after the cut-in point.

The safety valve and the low pressure warning system are driver-facing protections; the warning light and buzzer alert the driver before air pressure falls to a dangerous level.

The trailer hand valve (if equipped) should not be used for parking; it applies only the trailer brakes and can cause a jackknife if used while moving.

The low air pressure warning light and buzzer must activate before the pressure drops below 60 psi, giving the driver time to stop safely.

If the warning activates while driving, stop as soon as it is safe, park, and investigate; do not continue driving with low air pressure.

The spring brakes begin to apply automatically as pressure falls to about 20-45 psi, so the vehicle will stop on its own if the driver ignores the warning.

The pressure gauge should be checked frequently; a rapid drop in pressure while driving indicates a leak or a failed component.

The warning system is tested during the pre-trip inspection by pumping the brakes with the engine off until the light and buzzer come on.

A compressor that runs constantly or never cuts out indicates a governor or compressor problem; a compressor that never builds pressure indicates a failed compressor or a major leak.

The low pressure warning is a safety device, not a substitute for the pressure gauge; the driver must monitor both gauges and the warning system.

If the warning sounds and the spring brakes apply, the vehicle may be stopped in a dangerous location; use hazard flashers and warning triangles to alert other traffic.

A sudden loss of air pressure causes the spring brakes to apply automatically; the driver should hold the wheel, keep the vehicle straight, and let it slow while moving to the shoulder.

If the service brakes fail, the driver can use the parking brake (spring brakes) gradually to slow the vehicle, but must be careful not to lock the wheels and skid.

A leaking air line or fitting can often be identified by the sound of escaping air or by soapy water bubbles; the vehicle should not be driven until repaired.

A frozen air line or moisture in the system can block air flow; air dryers and alcohol evaporators help prevent freeze-up in cold weather.

A compressor failure means the system will not recharge; the vehicle can be driven only as far as the stored air allows, so stop and park safely.

A failed slack adjuster or brake chamber can cause a brake to drag or fail to release; a dragging brake may overheat and cause a fire or loss of braking.

If the brakes are wet, apply light pressure to dry them while driving slowly; wet brakes may pull to one side or fail to stop the vehicle.

If a tire blows out, hold the wheel firmly, avoid hard braking, and slow gradually; hard braking with a blown tire can cause loss of control.

In any emergency, the driver should warn other traffic with flashers and triangles, and call for assistance rather than attempting repairs on the roadway.