Georgia CDL Air Brakes Test flashcards

37 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 100 to 125 psi depending on the vehicle's specification. Above this point the air dryer usually purges.

Governor cut-in pressure

The pressure at which the governor restarts the compressor after cut-out, commonly about 85 psi. The cut-in to cut-out spread is normally 15 to 25 psi.

Air dryer function

Removes moisture and oil vapor from compressed air before it reaches the tanks. Its purge valve expels collected water at each compressor unload cycle; a saturated desiccant lets moisture pass through.

Supply (wet) tank

The first tank after the compressor and dryer. It collects condensation and feeds the other reservoirs; its manual drain cock must be opened to remove water.

One-way check valve

A valve between tanks that allows air to flow in only one direction. It keeps a failed compressor or broken line from draining stored pressure out of the other tanks.

Foot valve modulation

The foot or treadle valve delivers control pressure in proportion to how far it is pressed. Releasing it exhausts control air so the brakes release.

Relay valve purpose

A valve near the rear wheels that uses reservoir air to apply the brakes quickly when signaled by the foot valve, shortening lag time; failure causes slow or no rear application.

Slack adjuster stroke limit

The maximum pushrod travel allowed before brakes are considered out of adjustment. Excess stroke means the adjustment must be corrected before the vehicle is driven.

Spring brake chamber

A chamber containing a heavy coil spring that applies the brakes when air pressure is removed. Air pressure compresses the spring to release the brakes for normal driving.

Dual circuit purpose

Two independent air circuits with separate reservoirs, so a leak or failure in one circuit still leaves the other able to apply brakes and stop the vehicle.

Primary and secondary circuits

In the standard arrangement the primary circuit operates the rear brakes and the secondary circuit operates the front brakes. Each circuit has its own low pressure warning switch.

Low pressure warning threshold

The buzzer and light must activate before or when pressure in either circuit drops to 60 psi, and remain on until pressure is restored above the trigger point.

Parking brake valve

The yellow diamond push-pull knob. Pulling it out exhausts air to apply the spring brakes; pushing it in charges the spring chambers and releases them when pressure is adequate.

Tractor protection valve

Closes automatically when supply pressure falls to roughly 20 to 45 psi, sealing the tractor's air supply so a trailer leak or breakaway cannot drain the tractor's brakes.

Trailer supply valve

The red octagon knob controlling air to the trailer. It pops out automatically when tractor pressure falls into the protection range, applying the trailer spring brakes.

Front brake limiting valve

Where installed, reduces front brake force for slippery surfaces. It must be in the normal position for full braking on dry pavement.

Static leak test, released

Build to cut-out, shut off the engine, let pressure stabilize, then observe one minute. Loss must not exceed 3 psi on a single vehicle.

Static leak test, applied

With the engine off and pressure built, hold the foot brake for one minute. Allowable loss is 4 psi on a single vehicle and 6 psi on a combination vehicle.

Compressor build-up test

At operating rpm, pressure should rise from 85 to 100 psi within 45 seconds on a single vehicle. Slower build-up indicates a worn compressor, slipping belt, or leak.

Safety valve setting

A valve on the supply tank that opens at about 150 psi to prevent tank rupture if the governor fails and pressure climbs unchecked.

Tank draining procedure

Open all reservoir drain cocks to remove water and oil. Heavy or continuous water discharge suggests a failed air dryer or a missed draining schedule.

Spring brake pre-trip test

With pressure below 60 psi, push in the parking valve, place the transmission in gear, and confirm the vehicle does not move, proving the spring brakes apply.

Low pressure warning test

With the engine off, drain pressure slowly and watch for the buzzer and light. They must come on by 60 psi and stay on as pressure continues to fall.

Leak rate limits

Maximum allowable static loss is 3 psi per minute with brakes released and 4 psi applied on a single vehicle; combination vehicles allow 6 psi applied.

Governor cut-in/cut-out check

Note the pressure where the compressor stops and restarts. Cut-out is typically 100 to 125 psi, cut-in about 85 psi, with a 15 to 25 psi spread.

Air dryer purge check

Listen for a purge blast when the compressor reaches cut-out. No purge suggests a stuck purge valve or governor fault, letting moisture reach the tanks.

Failed check response

Any failed leak, warning, spring brake, or build-up check is an out-of-service defect. It must be corrected before the vehicle is placed in service.

Brake lag time

Air brakes react more slowly than hydraulic brakes because control pressure must travel to relay valves and pushrods must stroke before shoes contact the drums.

Downgrade braking technique

Select a low gear before descending, then use firm intermittent applications that slow the vehicle and release, allowing drums and shoes to cool between applications.

Brake fade cause

Continuous light brake pressure on a long grade overheats drums and linings, reducing friction so the brakes lose stopping power; gearing and intermittent braking prevent it.

Following distance rule

Allow at least one second per 10 feet of vehicle length plus one second. A 60-foot vehicle at 55 mph needs about 7 seconds.

Wet brake drying

After driving through deep water, apply the brakes lightly while moving slowly to dry the linings; wet brakes grab unevenly and pull to one side until dried.

Low pressure while driving

If the warning sounds or pressure falls below 60 psi, pull off the road and stop safely. At 20 to 45 psi the spring brakes apply automatically.

Failed compressor symptoms

Slow or no pressure build-up and pressure loss at idle. Downshift, conserve stored air, and stop before pressure is exhausted and the spring brakes apply.

Brake pull to one side

Indicates a stuck relay valve, collapsed hose, contaminated lining, or seized slack adjuster. Stop and inspect instead of steering to compensate.

Trailer breakaway response

The trailer spring brakes apply automatically through the emergency line, and the tractor protection valve closes to keep the tractor's air from escaping.

Total air loss response

Downshift progressively, use engine braking, and seek an escape ramp or safe runoff. Do not pump the pedal, because no stored air remains to apply the brakes.