Air Brake System Leak Checks and Pressure Tests
These tests prove the compressor, governor, warning devices and spring brakes work before you drive. Chock the wheels, release the parking brake, and start at full system pressure.
The sequence
- Static leak test: engine off, system at governor cut-out, brakes released. Watch the gauge for one minute. Loss should not exceed 3 psi on a single vehicle or 4 psi on a combination.
- Applied leak test: hold firm, steady pressure on the foot valve for one minute. Loss should not exceed 4 psi on a single vehicle or 6 psi on a combination. Check the current Massachusetts CDL manual for the exact figures used on your test.
- Low pressure warning: fan off pressure with the engine off. The light and buzzer must activate before 60 psi.
- Spring brake check: keep fanning. Spring brakes should apply in the 20-45 psi range.
- Build-up rate: at fast idle, pressure must climb from 85 to 100 psi within 45 seconds.
- Governor check: cut-out near 125 psi, cut-in near 100 psi.
Worked example
A combination holds 118 psi with the brakes released. After one minute the gauge reads 113 psi. Five psi exceeds the 4 psi combination limit, so there is a leak. Check the trailer lines, gladhands and wheel seals before dispatch.
Traps
Running the engine during the static test, forgetting to release the parking brake, timing only 30 seconds, and applying single-vehicle limits to a combination all cost marks. The static and applied tests have different limits, so do not swap them.