Georgia CDL Air Brakes Test cheat sheet

The compressor, driven by the engine, builds and maintains air pressure; a governor cuts the compressor in at roughly 85 psi and out at roughly 100-125 psi (cut-out pressure is stated on the vehicle's specification label).

The air dryer removes moisture and oil vapor downstream of the compressor; its purge valve releases collected water on each unload cycle, and a saturated desiccant cartridge passes moisture into the tanks.

The supply (wet) tank is the first tank after the dryer and collects condensation; the primary and secondary tanks feed the front and rear brake circuits respectively.

A one-way check valve between tanks prevents air from flowing back toward the compressor, so a failed compressor or broken line does not drain stored pressure from the other tanks.

The foot valve (brake valve) is a treadle-operated modulating valve: the further it is pressed, the higher the delivered control pressure, and releasing it exhausts control air to release the brakes.

Relay valves speed up application and release at the rear wheels by supplying reservoir air locally instead of routing all air through the foot valve; a failed relay valve causes slow or no rear brake application.

Slack adjusters convert pushrod stroke into brake shoe movement; a pushrod stroke beyond the adjustment limit means the brakes are out of adjustment and must be corrected before driving.

Spring (parking) brake chambers contain a heavy coil spring that applies the brakes when air pressure is removed; the spring is caged only for service or removal, never to roll with low pressure.

A dual air brake system has two separate circuits (primary and secondary), each with its own reservoir, so a leak or failure in one circuit leaves the other circuit able to stop the vehicle.

In the standard configuration the primary circuit operates the rear brakes and the secondary circuit operates the front brakes; each circuit has its own low-pressure warning switch.

A loss of pressure in one circuit should still leave at least the other circuit's brakes working; however, the vehicle's stopping distance will increase and the low pressure warning will activate.

The low pressure warning (buzzer and light) must activate before or when pressure in either circuit drops to 60 psi; the warning must stay on until pressure is restored above the trigger point.

The parking brake valve (yellow diamond knob) is a push-pull valve: pulled out it exhausts air and applies the spring brakes; pushed in it charges the spring chambers and releases them, provided system pressure is adequate.

The tractor protection valve closes automatically when supply pressure drops to about 20-45 psi, sealing the tractor's air supply so a trailer breakaway or trailer leak cannot drain the tractor's brakes.

The trailer supply valve (red octagon knob) controls air to the trailer; it pops out automatically when tractor pressure falls to the protection range, applying the trailer spring brakes.

A front brake limiting valve (where installed) reduces front brake force in slippery conditions; it must be in the normal position for full braking on dry pavement.

Build pressure to governor cut-out, then shut the engine off and check that pressure does not drop more than 3 psi in one minute with the brakes released (a single-vehicle leak test); a greater drop indicates a leak to locate and repair.

With pressure built and the engine off, apply and hold the foot brake for one minute: leakage must not exceed 4 psi per minute on a single vehicle, or 6 psi per minute on a combination vehicle.

Drain all air tanks fully at the end of the day (or during pre-trip when checking for water) and watch for moisture; a tank that discharges water continuously or heavily indicates a failed dryer or missing manual drain procedure.

Check the compressor build-up rate: with the engine at operating rpm, pressure should rise from 85 to 100 psi within 45 seconds on a single vehicle (or the manufacturer's stated time for a combination).

Verify the low pressure warning activates at or above 60 psi by draining pressure slowly with the engine off and observing the buzzer and light.

Test the spring brakes: with pressure below 60 psi, the parking brake valve pushed in, and the transmission in gear, the vehicle must not move; the spring brakes should apply automatically as pressure falls into the 20-45 psi range.

Check that the safety valve on the supply tank opens at about 150 psi; it is a backup that prevents tank rupture if the governor fails and pressure climbs unchecked.

Inspect the air dryer, drain cocks, lines, and fittings for cracks, chafing, and oil-soaked or weathered hose; check that all tanks have drain cocks and that the drain cocks open and close freely.

Static leak test, brakes released: build to cut-out, shut off engine, wait for stabilization, then observe for one minute; allowable loss is 3 psi maximum on a single vehicle.

Static leak test, brakes applied: hold the foot valve down for one minute; allowable loss is 4 psi on a single vehicle and 6 psi on a combination vehicle.

Low pressure warning test: with the engine off, pump or drain the service brakes until the warning buzzer and light come on; they must activate by 60 psi, and pressure must not continue to fall without the warning sounding.

Spring brake test: build full pressure, then drain the tanks until pressure is below 60 psi; the parking brake valve should pop out and the spring brakes should apply, and the vehicle must not roll when put in gear.

Compressor build-up test: at fast idle, time the pressure rise from 85 to 100 psi; it should take no more than 45 seconds on a single vehicle, and slower build-up points to a worn compressor, slipping belt, or leak.

Governor cut-out and cut-in check: note the pressure at which the compressor stops (cut-out) and restarts (cut-in); cut-out is typically 100-125 psi and cut-in about 85 psi, and the spread should be roughly 15-25 psi.

Air dryer purge check: listen for the purge blast at cut-out; no purge indicates a stuck purge valve or governor problem, which allows moisture to reach the tanks.

If any check fails (excess leak rate, late warning, no spring brake application, slow build-up), the defect must be corrected before the vehicle is placed in service.

Allow the system to build to governor cut-out before releasing the parking brake and moving; the spring brakes will not release fully until supply pressure is high enough to compress the springs.

Use the brakes with the transmission in a gear appropriate to the grade; on long downgrades select a low gear before descending and use intermittent braking rather than continuous light pressure.

Continuous light brake application on a long grade overheats drums and shoes, causing brake fade; a firm application that slows the vehicle, then release, lets the brakes cool between applications.

Braking distance for air brakes includes lag time: control pressure must travel to the relay valves and pushrods must stroke before shoes contact drums, so air brakes react more slowly than hydraulic brakes.

At highway speed, maintain a following distance of at least one second per 10 feet of vehicle length plus one second; for a 60-foot vehicle at 55 mph that is about 7 seconds.

Stab braking (hard application to slow, then release) is the recommended technique for a vehicle with air brakes on a long downgrade when speed builds despite gearing; do not fan or pump the pedal lightly.

The parking brake valve must be pushed in for normal driving and pulled out to park; never use the parking brake as a service brake at speed, as it applies only the spring brakes abruptly.

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

If the low pressure warning sounds or the gauge falls below 60 psi while driving, move off the road and stop as soon as it is safe; continued driving risks automatic spring brake application at 20-45 psi.

If the spring brakes apply automatically at low pressure, the vehicle will stop on its own; steer to the shoulder, stop, and do not attempt to drive until pressure is restored and the cause corrected.

A failed compressor shows as slow or no pressure build-up and rising pressure loss when the engine is idling; downshift, use the remaining stored air sparingly, and stop before pressure is exhausted.

A brake pull to one side during application indicates a stuck relay valve, collapsed hose, contaminated lining, or seized slack adjuster; stop and inspect rather than compensating with steering.

A spongy or low pedal with air brakes usually indicates air in the hydraulic section (on air-over-hydraulic systems) or a failed foot valve; do not continue driving with an abnormal pedal feel.

If a trailer breaks away, the trailer spring brakes apply automatically through the emergency line; the tractor protection valve closes and keeps the tractor's air from escaping.

A stuck purge valve or failed dryer causes water carryover; in freezing weather this can freeze lines and cause brake drag or failure, so drain tanks and repair the dryer before operating in cold conditions.

If brakes fail completely, downshift progressively, use engine braking, and look for an escape ramp or a safe runoff area; do not pump the pedal in a total air loss because no stored air remains to apply the brakes.

The compressor, driven by the engine, builds and maintains air pressure; a governor cuts the compressor in at roughly 85 psi and out at roughly 100-125 psi (cut-out pressure is stated on the vehicle's specification label).

The air dryer removes moisture and oil vapor downstream of the compressor; its purge valve releases collected water on each unload cycle, and a saturated desiccant cartridge passes moisture into the tanks.

The supply (wet) tank is the first tank after the dryer and collects condensation; the primary and secondary tanks feed the front and rear brake circuits respectively.

A one-way check valve between tanks prevents air from flowing back toward the compressor, so a failed compressor or broken line does not drain stored pressure from the other tanks.

The foot valve (brake valve) is a treadle-operated modulating valve: the further it is pressed, the higher the delivered control pressure, and releasing it exhausts control air to release the brakes.

Relay valves speed up application and release at the rear wheels by supplying reservoir air locally instead of routing all air through the foot valve; a failed relay valve causes slow or no rear brake application.

Slack adjusters convert pushrod stroke into brake shoe movement; a pushrod stroke beyond the adjustment limit means the brakes are out of adjustment and must be corrected before driving.

Spring (parking) brake chambers contain a heavy coil spring that applies the brakes when air pressure is removed; the spring is caged only for service or removal, never to roll with low pressure.

A dual air brake system has two separate circuits (primary and secondary), each with its own reservoir, so a leak or failure in one circuit leaves the other circuit able to stop the vehicle.

In the standard configuration the primary circuit operates the rear brakes and the secondary circuit operates the front brakes; each circuit has its own low-pressure warning switch.

A loss of pressure in one circuit should still leave at least the other circuit's brakes working; however, the vehicle's stopping distance will increase and the low pressure warning will activate.

The low pressure warning (buzzer and light) must activate before or when pressure in either circuit drops to 60 psi; the warning must stay on until pressure is restored above the trigger point.

The parking brake valve (yellow diamond knob) is a push-pull valve: pulled out it exhausts air and applies the spring brakes; pushed in it charges the spring chambers and releases them, provided system pressure is adequate.

The tractor protection valve closes automatically when supply pressure drops to about 20-45 psi, sealing the tractor's air supply so a trailer breakaway or trailer leak cannot drain the tractor's brakes.

The trailer supply valve (red octagon knob) controls air to the trailer; it pops out automatically when tractor pressure falls to the protection range, applying the trailer spring brakes.

A front brake limiting valve (where installed) reduces front brake force in slippery conditions; it must be in the normal position for full braking on dry pavement.

Build pressure to governor cut-out, then shut the engine off and check that pressure does not drop more than 3 psi in one minute with the brakes released (a single-vehicle leak test); a greater drop indicates a leak to locate and repair.

With pressure built and the engine off, apply and hold the foot brake for one minute: leakage must not exceed 4 psi per minute on a single vehicle, or 6 psi per minute on a combination vehicle.

Drain all air tanks fully at the end of the day (or during pre-trip when checking for water) and watch for moisture; a tank that discharges water continuously or heavily indicates a failed dryer or missing manual drain procedure.

Check the compressor build-up rate: with the engine at operating rpm, pressure should rise from 85 to 100 psi within 45 seconds on a single vehicle (or the manufacturer's stated time for a combination).

Verify the low pressure warning activates at or above 60 psi by draining pressure slowly with the engine off and observing the buzzer and light.

Test the spring brakes: with pressure below 60 psi, the parking brake valve pushed in, and the transmission in gear, the vehicle must not move; the spring brakes should apply automatically as pressure falls into the 20-45 psi range.

Check that the safety valve on the supply tank opens at about 150 psi; it is a backup that prevents tank rupture if the governor fails and pressure climbs unchecked.

Inspect the air dryer, drain cocks, lines, and fittings for cracks, chafing, and oil-soaked or weathered hose; check that all tanks have drain cocks and that the drain cocks open and close freely.

Static leak test, brakes released: build to cut-out, shut off engine, wait for stabilization, then observe for one minute; allowable loss is 3 psi maximum on a single vehicle.

Static leak test, brakes applied: hold the foot valve down for one minute; allowable loss is 4 psi on a single vehicle and 6 psi on a combination vehicle.

Low pressure warning test: with the engine off, pump or drain the service brakes until the warning buzzer and light come on; they must activate by 60 psi, and pressure must not continue to fall without the warning sounding.

Spring brake test: build full pressure, then drain the tanks until pressure is below 60 psi; the parking brake valve should pop out and the spring brakes should apply, and the vehicle must not roll when put in gear.

Compressor build-up test: at fast idle, time the pressure rise from 85 to 100 psi; it should take no more than 45 seconds on a single vehicle, and slower build-up points to a worn compressor, slipping belt, or leak.

Governor cut-out and cut-in check: note the pressure at which the compressor stops (cut-out) and restarts (cut-in); cut-out is typically 100-125 psi and cut-in about 85 psi, and the spread should be roughly 15-25 psi.

Air dryer purge check: listen for the purge blast at cut-out; no purge indicates a stuck purge valve or governor problem, which allows moisture to reach the tanks.

If any check fails (excess leak rate, late warning, no spring brake application, slow build-up), the defect must be corrected before the vehicle is placed in service.

Allow the system to build to governor cut-out before releasing the parking brake and moving; the spring brakes will not release fully until supply pressure is high enough to compress the springs.

Use the brakes with the transmission in a gear appropriate to the grade; on long downgrades select a low gear before descending and use intermittent braking rather than continuous light pressure.

Continuous light brake application on a long grade overheats drums and shoes, causing brake fade; a firm application that slows the vehicle, then release, lets the brakes cool between applications.

Braking distance for air brakes includes lag time: control pressure must travel to the relay valves and pushrods must stroke before shoes contact drums, so air brakes react more slowly than hydraulic brakes.

At highway speed, maintain a following distance of at least one second per 10 feet of vehicle length plus one second; for a 60-foot vehicle at 55 mph that is about 7 seconds.

Stab braking (hard application to slow, then release) is the recommended technique for a vehicle with air brakes on a long downgrade when speed builds despite gearing; do not fan or pump the pedal lightly.

The parking brake valve must be pushed in for normal driving and pulled out to park; never use the parking brake as a service brake at speed, as it applies only the spring brakes abruptly.

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

If the low pressure warning sounds or the gauge falls below 60 psi while driving, move off the road and stop as soon as it is safe; continued driving risks automatic spring brake application at 20-45 psi.

If the spring brakes apply automatically at low pressure, the vehicle will stop on its own; steer to the shoulder, stop, and do not attempt to drive until pressure is restored and the cause corrected.

A failed compressor shows as slow or no pressure build-up and rising pressure loss when the engine is idling; downshift, use the remaining stored air sparingly, and stop before pressure is exhausted.

A brake pull to one side during application indicates a stuck relay valve, collapsed hose, contaminated lining, or seized slack adjuster; stop and inspect rather than compensating with steering.

A spongy or low pedal with air brakes usually indicates air in the hydraulic section (on air-over-hydraulic systems) or a failed foot valve; do not continue driving with an abnormal pedal feel.

If a trailer breaks away, the trailer spring brakes apply automatically through the emergency line; the tractor protection valve closes and keeps the tractor's air from escaping.

A stuck purge valve or failed dryer causes water carryover; in freezing weather this can freeze lines and cause brake drag or failure, so drain tanks and repair the dryer before operating in cold conditions.

If brakes fail completely, downshift progressively, use engine braking, and look for an escape ramp or a safe runoff area; do not pump the pedal in a total air loss because no stored air remains to apply the brakes.