Trucker Tools
CDL Air Brake Test: 7-Step Air Brake Check
The CDL air brake check has seven parts: air leakage rate, low-air warning, automatic spring brakes, pressure buildup, governor cut-out and cut-in, parking brake and service brakes. A combination vehicle must lose less than 3 psi in one minute with the brakes released, and no more than 4 psi with the foot brake applied.
The 7 steps
- Test the air leakage rate. With a fully charged system (typically 125 psi), turn the engine off, release the parking brake and time the drop for 1 minute: less than 2 psi for a single vehicle, less than 3 psi for a combination. Then hold 90 psi or more on the brake pedal: after the initial drop, more than 3 psi (single) or 4 psi (combination) in 1 minute is too much.
- Test the low-air warning. Engine off with enough pressure that the warning is off, key on, fan the brake pedal. The warning must come on before pressure drops below 60 psi (in the tank with the lowest pressure on dual systems).
- Check that the spring brakes come on automatically. Level ground, wheels chocked, parking brakes released, engine off, keep fanning the pedal. The tractor protection valve and parking brake valve should pop out at the manufacturer's specification (the manual lists 40 to 20 psi).
- Check the rate of air pressure buildup. At operating rpm, pressure should build from 85 to 100 psi within 45 seconds on dual air systems. Larger than minimum tanks can take longer and still be safe; check the manufacturer's specification.
- Check governor cut-out and cut-in. Pumping should stop at about 125 psi (cut-out) and start again at about 100 psi (cut-in). Run at fast idle to see cut-out, then fan the pedal to see cut-in. Check the manufacturer's specification.
- Test the parking brake. Stop, set the parking brake and gently pull against it in a low gear to make sure it holds.
- Test the service brakes. With normal air pressure, release the parking brake, move forward slowly (about 5 mph) and brake firmly. Note any pulling to one side, unusual pedal feel or delayed stopping.
The pass numbers
| Check | Single vehicle | Combination |
|---|---|---|
| Leak, brakes released, engine off, 1 minute | less than 2 psi | less than 3 psi |
| Leak, foot brake at 90 psi or more, 1 minute after initial drop | 3 psi or less | 4 psi or less |
| Low-air warning | on before pressure drops below 60 psi | |
| Spring brakes / valve pop-out | manufacturer spec (manual lists 40 to 20 psi) | |
| Buildup, dual systems, operating rpm | 85 to 100 psi within 45 seconds | |
| Governor | cut-in about 100 psi, cut-out about 125 psi (check manufacturer spec) | |
Numbers from the AAMVA-based state CDL manual (PennDOT Pub 223 section 5; the Utah CDL handbook section 5 uses the same text). Use your state's manual.
Federal warning-device rule versus CDL manual guidance
The leak rates, buildup time and governor figures above are CDL-manual guidance. For trucks, truck tractors and buses manufactured on or after March 1, 1975, 49 CFR 393.51(c) requires a warning device meeting the applicable FMVSS 121 requirements. FMVSS 121 S5.1.5 calls for a continuous warning when the ignition is on and service-reservoir pressure is below 60 psi. For power units not subject to FMVSS 121 at manufacture, 393.51(c)(2) instead specifies a warning at 55 psi and below, or one-half of governor cut-out pressure, whichever is less. Use your state CDL manual for the test.
Class A add-on checks
Practice questions: your state CDL manual and its testing site have the official air brakes knowledge material. The FMCSA CDL manual page points you to your state's manual.
FAQ
What is the air leakage limit on the CDL air brake test?
With the engine off and brakes released, the loss must be under 2 psi in one minute for a single vehicle and under 3 psi for a combination. With 90 psi or more held on the brake pedal, after the initial drop, no more than 3 psi (single) or 4 psi (combination) a minute (CDL manual section 5).
When must the low-air warning come on?
The low air pressure warning must come on before pressure in the air tank drops below 60 psi. On a dual air system, watch the tank with the lowest pressure. If the warning does not work, you could lose air pressure without knowing it, so get it fixed before you drive (CDL manual section 5).
When do the spring brakes come on?
At the manufacturer's specification. In the air brake check the CDL manual lists the parking brake valve popping out at 40 to 20 psi, and its low-air section says spring brakes come on when pressure drops into the range of 20 to 45 psi. Know your own truck's number and use your state manual.
How fast should air pressure build up?
On a dual air system at operating rpm, pressure should build from 85 to 100 psi within 45 seconds. Trucks with larger than minimum air tanks can take longer and still be safe, so check the manufacturer's specification. If pressure builds too slowly, do not drive until it is fixed (CDL manual section 5).
What are governor cut-in and cut-out pressures?
The CDL manual says the compressor should start pumping at about 100 psi (cut-in) and stop at about 125 psi (cut-out), and to check the manufacturer's specification. Its in-cab section mentions cut-out usually around 120 to 140 psi. Use the number for your truck and your state's manual, not one fixed figure.
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Sources
- PennDOT Pub 223 CDL Manual, Section 5 (air brakes)
- PennDOT Pub 223 CDL Manual, Section 6 (combination vehicles)
- Utah CDL Handbook, Section 5 (air brakes)
- 49 CFR 383.113, Required skills (eCFR)
- FMCSA, CDL Manual (use your state's manual)
- 49 CFR 393.51, Warning signals, air pressure and vacuum gauges (eCFR)
- 49 CFR 571.121, FMVSS 121, S5.1.5 warning signal (eCFR)
Disclaimer: For general information. Federal limits from 23 CFR 658 and 49 CFR; state limits, permits and enforcement vary. Check your state's CDL manual and DOT before you rely on a number. Not legal or tax advice.
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