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Georgia CDL Air Brakes Test study materials
Georgia CDL Air Brakes Test study resources

Georgia CDL Air Brakes Test: Air Brake System Checks (Leak Rate, Low Pressure Warning, Spring Brake)

Georgia CDL Air Brakes Test · Work through your exam, one topic at a time.

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Air Brake System Checks (Leak Rate, Low Pressure Warning, Spring Brake)

Domain 4 is the quantitative heart of the air brake test: leak rate, low-pressure warning, spring brake application, and compressor build. The Georgia CDL exam often asks for the pass or fail threshold and the correct test sequence.

Thresholds and sequence

  • Static leak test: fully charged system, engine off, parking brake released, service brake applied and held for one minute. Maximum loss is 3 psi for a single vehicle and 4 psi for a combination. An unapplied leak check may also be required; follow the vehicle manual.
  • Low-pressure warning: with the engine off, pump the service brake to reduce pressure. The warning light or buzzer must activate before pressure drops below 60 psi.
  • Spring brake test: continue reducing pressure. Spring brakes must apply between 20 and 45 psi. If they do not, the vehicle fails.
  • Compressor build: with the engine at operating RPM, pressure should build from 85 to 100 psi within 45 seconds. Governor cut-out is about 120-135 psi; cut-in is about 100 psi.

Worked example

A combination vehicle is charged to 120 psi. Engine off, parking brake released, service brake held: loss is 4 psi in one minute, which passes because the limit is 4 psi for a combination. Pumping down, the warning sounds at 58 psi, which fails because it must activate before 60. Spring brakes apply at 30 psi, which passes. Build from 85 to 100 psi takes 40 seconds, which passes.

Traps that cost marks

Do not use idle RPM for the build test; use operating RPM. Do not test the low-pressure warning with the engine running, because the compressor will fight the pressure drop. Do not confuse the safety valve at about 150 psi with governor cut-out. Do not accept a warning at 58 psi or spring brake application at 15 psi. Do not apply the parking brake during the service-brake leak test.

Key points

  • Leak test: engine off, parking brake released, service brake held.
  • Limit: 3 psi single, 4 psi combination.
  • Warning must activate before 60 psi.
  • Spring brakes must apply at 20-45 psi.
  • Build test uses operating RPM, not idle.