LA CDL Air Brakes flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Governor cut-out pressure

The pressure at which the governor unloads the compressor, typically about 120-125 psi. Above this point the compressor stops building pressure and the system holds steady.

Governor cut-in pressure

The pressure at which the governor reloads the compressor after it has unloaded, typically about 100 psi. The compressor then runs until cut-out is reached again.

Supply (wet) reservoir

The first tank after the compressor. It collects moisture and oil from the compressed air and feeds the other reservoirs through one-way check valves.

One-way check valve

A valve that lets air flow into a reservoir but not back out. It keeps air from escaping if the compressor or supply tank fails.

Foot (treadle) valve

The cab pedal valve that delivers air to the brake chambers in proportion to pedal travel. It is a graduated control, not a simple on-off switch.

Slack adjuster

The lever that converts brake chamber pushrod stroke into rotation of the brake camshaft. It compensates for lining wear and must stay within its adjustment limit.

Spring brake chamber

A chamber containing a strong spring that applies the brakes when air pressure is released. Air holds the spring off, so losing air applies the brakes.

Low-pressure warning switch

A switch that activates a buzzer and/or light when system pressure falls below about 60 psi, warning the driver that braking power is being lost.

Dual-circuit air system

Two separate air circuits, such as front and rear axles, sharing one foot valve but having separate reservoirs. A failure in one circuit leaves the other pressurized.

Static leak test

With the engine off and the system fully charged, hold the foot brake fully applied for one minute. A loss over 3 psi (single) or 4 psi (dual) fails.

Applied leak test

Charge the system, release the parking brake, and hold the foot brake fully applied for one minute. A drop over 3 psi (single) or 4 psi (dual) indicates a leak.

Low-pressure warning test

With the engine off, fan the brakes to reduce pressure. The warning buzzer or light must activate by the time pressure falls to about 60 psi.

Spring brake test

Reduce pressure below about 60 psi with the engine off. The parking brake valve must pop out and the spring brakes must apply; if not, the test fails.

Compressor build-up test

At fast idle, pressure should rise from about 85 psi to 100 psi in about 45 seconds in a typical system. Check the manufacturer's specification for the exact limit.

Pushrod travel limit

The maximum brake chamber pushrod stroke marked on the chamber (often about 1 inch for a type 30 chamber). Travel beyond the limit means the brake needs service.

Reservoir draining

Opening the drain valves at the end of the day removes moisture and oil from the tanks. Large amounts of water or oil indicate a compressor or air dryer problem.

Air brake lag

The delay while air pressure travels from the foot valve to the brake chambers. It makes stopping distance longer than with hydraulic brakes, so allow extra following distance.

Brake fade

Loss of braking power when the brakes overheat from continuous use. Gearing down and using engine braking on long downgrades reduces the risk.

Parking brake valve

The yellow diamond knob that releases the spring brakes when pushed in and applies them when pulled out. It cannot be released below about 60 psi.

Air dryer

A device that removes moisture and oil from compressed air before it reaches the reservoirs. A failed dryer lets water into the system, where it can freeze and block valves.

Fanning the brakes

Repeatedly pumping the brake pedal. Each application uses stored air, and fanning can exhaust the reservoirs faster than the compressor can recharge them.

Engine braking on downgrades

Selecting a low gear so the engine and brakes together hold the speed on a long steep grade. Do not rely on the foot brake alone, which can overheat.

Single-circuit failure

If one circuit of a dual system fails, the other still brakes, but stopping distance increases and the low-pressure warning may activate. Stop and check the system.

Low-pressure warning meaning

A buzzer or light at about 60 psi means the system is losing pressure. Stop as soon as it is safe and do not continue driving.

Spring brakes applying by themselves

The parking brake valve pops out when pressure is lost, and the spring brakes stop the vehicle. It cannot be moved again until pressure is restored.

Burst hose or line

A rapid pressure loss with a loud hiss usually means a burst hose, a failed chamber diaphragm, or a broken line. Stop and do not attempt to drive.

Foot brake failure response

Use the engine brake, downshift through the gears, and apply the parking brake gradually. A sudden full spring brake application at speed can cause a skid.

Dragging brake signs

One hot wheel, pulling to one side, or a burning smell indicates a dragging brake that can overheat and fade. Stop, let it cool, and correct the cause.

Compressor failure

If the compressor stops building air, only the stored air remains. Use the brakes sparingly and stop at the first safe place.

Leak detection with soap

A soap solution applied to a hose, fitting, or chamber joint bubbles at the leak point. A leak that can be heard or felt is a defect that needs repair.

Slack adjuster misadjustment

Adjusting the slack adjuster to hide worn linings or excessive pushrod travel is wrong. Pushrod travel beyond the limit means the foundation brake needs service.

Water or oil in drained air

Water or oil in the air drained from the reservoirs indicates that the air dryer or the compressor is failing. It needs service before the system is used again.