OK 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 to 125 psi on most heavy-duty air brake systems.

Governor cut-in pressure

The pressure at which the governor restarts the compressor after cut-out, usually near 100 psi, restoring reservoir pressure for the next brake application.

Air dryer purge cycle

At compressor cut-out, the air dryer ejects collected moisture and oil through its drain valve, protecting downstream reservoirs and valves from contamination.

Wet tank

The first reservoir to receive compressor air; it collects moisture and is fitted with a drain cock and the system safety valve.

Safety valve setting

A spring-loaded valve on the wet tank that opens near 150 psi to vent excess pressure if the governor fails to cut out the compressor.

Slack adjuster function

A lever on the brake chamber pushrod that multiplies force and converts pushrod stroke into S-cam rotation to apply the brake shoes.

Spring brake release pressure

Spring brakes require roughly 90 to 100 psi of air to fully compress the springs and release; below that they apply or drag.

Low-pressure warning threshold

A buzzer and warning light must activate before system pressure drops below 60 psi, alerting the driver to an impending brake loss.

Dual circuit independence

Primary and secondary circuits each have their own reservoir and check valve, so a leak or failure in one does not drain the other.

Check valve purpose

A one-way valve that lets air into a reservoir but blocks backflow, preventing a failed circuit from bleeding pressure out of the other circuit.

Tractor protection valve

A valve that closes the tractor-to-trailer air lines when tractor pressure falls to about 20 to 45 psi, preserving tractor brake pressure.

Trailer hand valve

A dash-mounted lever (Johnson bar) that applies trailer brakes only; it must not be used for parking because it can leak off.

Front brake limiting valve

A valve that reduces front axle braking force in the bobtail condition to lower the risk of front wheel lockup and loss of steering.

Parking brake control knob

The yellow diamond knob that releases air from the spring brake chambers; pulling it applies the spring brakes for parking.

Pressure buildup test

With the engine at operating rpm, pressure should climb from 85 to 100 psi within about 45 seconds on a typical tractor-trailer.

Static leak test

With the engine off and full pressure, release the service brake and confirm pressure drops no more than 3 psi in one minute.

Applied leak test

Fully apply and hold the foot valve; leakage must not exceed 4 psi per minute for a single vehicle or 6 psi for a combination.

Spring brake pop-out test

Pump the foot valve to lower pressure; below 60 psi the parking knob should pop out and the spring brakes should apply automatically.

Pushrod travel check

Measure slack adjuster pushrod stroke against the manufacturer's limit; excessive travel signals misadjustment or worn brake components.

Reservoir draining

Open drain cocks at the low point of each tank at day's end; water, oil, or milky fluid indicates dryer or compressor trouble.

Continuous low-pressure alarm

A red warning light and buzzer that stay on below 60 psi mean the driver must stop and locate the leak before continuing.

Excessive leak rate

Pressure falling faster than 3 psi per minute unapplied, or 4 psi per minute applied, indicates a leak that must be repaired.

Oil in reservoir drains

Compressor oil passing into the air system or a failed air dryer contaminates valves and seals and can cause brake drag or failure.

Brake pull to one side

A vehicle that pulls during braking likely has contaminated linings, a stuck S-cam, or a seized slack adjuster on one side.

Brake fade

Heat-induced loss of friction in the drums and linings, caused by continuous light brake application on long downgrades.

Snub braking

Apply the brakes firmly to shed about 5 to 10 mph, then release and let the drums cool before the next application on a grade.

Total stopping distance

Perception distance plus reaction distance plus braking distance; braking distance grows with the square of speed.

Wet brake drying

After driving through deep water, apply light brake pressure at low speed to heat and dry the linings before relying on them.

Engine braking on grades

Selecting a low gear before a downgrade and using engine or retarder braking controls speed and reduces reliance on the service brakes.

Service brake failure response

Downshift progressively, use engine braking, and apply the parking brake gradually to avoid locking the rear wheels and jackknifing.

Escape ramp use

On a steep downgrade with failed brakes, steer straight into the gravel escape bed and expect a sudden, firm stop.

Air brake restriction L

A CDL restriction that prohibits operating a CMV with air brakes; it is removed only by passing the air brake knowledge and skills tests.