OR 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 stops the compressor from building more air, typically around 120-125 psi on many heavy vehicles. Above this, the compressor unloads.

Governor cut-in pressure

The pressure at which the governor restarts the compressor after cut-out, typically about 100 psi. It keeps the system charged within a set range.

Wet tank

The supply reservoir that receives air directly from the compressor and condenses moisture before air moves to the other tanks. It should be drained regularly.

Air dryer function

A device after the compressor that removes moisture and oil vapor from compressed air before it reaches the reservoirs, helping prevent corrosion and frozen lines.

Treadle (foot) valve

The driver's service brake control. It meters compressed air to the brake chambers in proportion to how far the pedal is pushed.

Slack adjuster

The linkage that transmits pushrod force to the brake shoe and provides adjustment. Too much pushrod travel means the brakes are out of adjustment.

Safety valve

A valve on the air tank, usually set near 150 psi, that releases air to protect the system from overpressure if the governor fails.

Spring brake chamber

A brake chamber with a mechanically stored spring that applies the brake when air pressure is released. It provides parking and emergency braking.

Dual air brake system

A split system with two separate circuits, usually front and rear, each with its own reservoir and check valve, so one failure does not drain both.

Check valve purpose

A one-way valve that prevents air from flowing back out of a reservoir, so a leak in one circuit cannot drain the other circuit's stored air.

Low pressure warning threshold

The warning light and buzzer must activate before service brake pressure drops below 60 psi, alerting the driver to a dangerous loss of air.

Spring brake default state

Spring brakes are applied by spring force and held off by air pressure. Loss of air pressure causes them to apply automatically as a fail-safe.

Parking brake control

The yellow diamond knob. Pulling it out releases air to apply the spring brakes; pushing it in with sufficient pressure releases them.

Tractor protection valve

A valve on combination vehicles that protects the tractor air supply by closing off a leaking or broken trailer circuit, preserving tractor braking.

Trailer supply valve

The red knob that controls air to the trailer. It pops out automatically when tractor pressure drops too low, applying the trailer spring brakes.

Compressor belt check

During inspection, check the compressor drive belt for cracks, glazing, and correct tension. A slipping belt prevents the system from building enough pressure.

Tank draining

Drain all air tanks regularly to remove moisture and oil. The wet tank should be drained first, and moisture carryover suggests the air dryer needs service.

Brake chamber inspection

Check brake chambers for dents, cracks, and loose clamps. The pushrod should move freely, and the chamber should not be damaged or leaking air.

Pushrod travel check

With the brakes applied, measure pushrod travel. Travel beyond the manufacturer's limit means the brakes are out of adjustment and need service.

Lining and drum check

Inspect brake linings and drums for wear, cracks, and oil or grease contamination. Contaminated linings reduce friction and stopping power.

Air line inspection

Check all air lines and hoses for chafing, cracks, and leaks, and confirm gladhands and couplers are sealed and secure before driving.

Static leak test

With the engine off and the system charged, release the parking brake and hold the service brake. Pressure should not drop more than about 3 psi in one minute.

Applied leak test

With the system charged and the engine off, apply and hold the service brake. A steady pressure drop indicates a leak in the service brake system.

Low pressure warning test

With the engine off, pump the brake pedal to reduce pressure. The warning light and buzzer must come on before pressure drops below 60 psi.

Spring brake function test

With the system charged, pull the parking brake knob to apply the spring brakes. Push it in with sufficient pressure and they should release.

Automatic spring brake test

Pump the brake pedal until the parking brake knob pops out automatically, showing the spring brakes applied because pressure fell too low.

Build-up check

With the system at low pressure and the engine at fast idle, pressure should build from 85 to 100 psi within about 45 seconds, per the manufacturer's spec.

Air brake lag

There is a delay while air travels through the lines, so air brakes do not apply instantly. Plan longer stopping distances than with hydraulic brakes.

Downgrade braking technique

Select a low gear before a long or steep downgrade and use engine braking. Use short, firm brake applications rather than continuous light drag.

Brake fade

Overheated brakes lose friction and stopping power, usually from continuous light drag on a downgrade. Release the brakes and let them cool.

Pressure gauge monitoring

Watch the air pressure gauge continuously, especially on downgrades. If pressure drops, stop and find the cause before continuing.

Service brake failure response

If service brakes fail, spring brakes apply automatically when pressure drops low enough. Downshift for engine braking and steer toward a safe escape route.