Colorado 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 so it stops pumping air. Typical value is near 125 psi, and continued pressure rise beyond that suggests a governor or safety-valve fault.

Safety valve setting

A valve on the supply reservoir that vents air to the atmosphere if pressure climbs too high, normally near 150 psi. It protects the system if the governor fails to unload the compressor.

Graduated foot valve

The treadle valve that admits air in proportion to pedal travel. Light pressure produces light braking and harder pressure produces harder braking, so the driver controls braking force continuously rather than on and off.

Dual-circuit air system

A system with separate primary and secondary circuits fed through one-way check valves. If one circuit fails, the other still provides braking, and a low-pressure warning alerts the driver.

Spring brake operation

A brake chamber that applies by stored spring force when air pressure is released and releases only when air pressure is restored. It provides the parking brake and emergency stopping ability.

Alcohol evaporator purpose

A device that adds alcohol vapor to the air system to prevent moisture from freezing in valves and lines during cold weather. An air dryer serves a similar moisture-removal purpose.

Static leak test

With a fully charged system, engine off, and brakes released, watch the gauge for one minute. A loss of more than 2 psi in one minute indicates a leak.

Applied leak test

With the system charged, hold the foot valve fully down for one minute. A pressure loss of more than 3 psi in one minute is excessive and means the vehicle needs repair.

Low-pressure warning test

With the engine off, pump the foot valve until the warning light and buzzer activate. They must come on before pressure drops below 60 psi, confirming the warning system works.

Spring brake test

With the system charged, pull the yellow knob and fan the foot valve to bleed air. The spring brakes must set, preventing movement, by the time pressure falls to about 45 psi.

Compressor build-up test

With the engine at operating rpm and the system drained, pressure should rise from 85 to 100 psi in roughly 45 seconds. A common acceptable range is about 30 to 60 seconds.

Reservoir draining

Opening the drain cocks on the air reservoirs to remove moisture and oil carryover, usually daily. Water in the system can freeze in cold weather and cause brake failure.

Slack adjuster free play

Pulling a slack adjuster by hand should show little movement. More than about 1 inch of free play at the push rod means the brake needs adjustment or repair.

Push-rod stroke limit

With the brakes fully applied, push-rod travel should not exceed about 1 inch per 10 psi of application pressure. Too long a stroke means the chamber runs out of travel before full braking.

Automatic slack adjusters

Slack adjusters that adjust themselves as brakes wear. They should not be manually adjusted to hide a chronic problem; the underlying cause, such as a bad chamber, must be found.

Low-pressure warning device

A light and buzzer that activate when system pressure falls below about 60 psi. They warn the driver that braking force is being lost and that a safe stop is required.

Brake fade

Loss of stopping power caused by overheated drums and linings, common on long downgrades when brakes are used continuously. Using a lower gear and engine braking helps prevent it.

Dragging brake symptoms

A brake that stays partly applied shows a hot drum, smoke, or a pull to one side. Causes include a stuck slack adjuster, a broken spring, or a seized S-cam.

Pressure will not build

If the gauge does not reach normal pressure or falls rapidly while driving, the brakes may fail completely. Stop safely and do not continue until the system is repaired.

Parking brake control

The yellow diamond knob that controls the spring brakes. Pull it to apply the parking brake and push it to release, and it stays applied without air pressure.

Parking brake while moving

Do not use the parking brake for a normal stop at speed, except in an emergency. Routine braking uses the foot valve, which gives graduated control.

Downgrade braking technique

Select a low gear before starting down a long grade and use engine braking to control speed. The foot brake is for control, not for holding speed continuously.

Service brake failure response

If the service brakes fail, apply the parking spring brake gradually, downshift, and look for a runaway ramp or safe escape route rather than continuing at speed.

Following distance

An air-braked combination vehicle needs far more stopping distance than a car because of its weight and speed, so the driver should increase following distance accordingly.

Parking on a grade

Set the parking brake and chock the wheels when parked on a slope. A spring brake can release accidentally if air pressure is restored, so chocking adds protection.

Cold weather air brakes

Drain moisture and let pressure build fully before moving. Frozen lines or valves can prevent the brakes from releasing or applying, so check the system before driving.

Cooling hot brakes

After a hard stop or long downgrade, let the brakes cool before parking. Setting the parking brake on hot drums can cause cracking or fade on the next use.

Low-air warning response

When the low-air warning activates, stop as soon as it is safe, set the parking brake, and do not move the vehicle until the system is repaired and pressure is normal.

Compressor function

The engine-driven compressor supplies air to the reservoirs but does not store air itself. The governor controls when it pumps and when it unloads.

One-way check valves

Valves that let air flow into the primary and secondary circuits but not back out. They keep a failure in one circuit from draining pressure from the other.

Air dryer purpose

A device that removes moisture from compressed air before it reaches the reservoirs. Keeping the system dry prevents freeze-ups in cold weather and reduces corrosion and valve damage.

Governor cut-in pressure

The pressure at which the governor lets the compressor resume pumping after it has unloaded, typically near 85 psi. It keeps reservoir pressure within a safe operating band.