Normal Operating Checks and Air Pressure Tests
Build-up and cut-out
Start with the system at rest, run the engine, and watch the gauges build. Normal build-up is quick - roughly from 85 to 100 psi in about 45 seconds for a typical tractor-trailer. The governor should cut the compressor out near the top of the range and back in near the bottom. If pressure will not build, or builds very slowly, stop - the vehicle is not safe.
Static leak test
With the system fully charged, shut off the engine, let the pressure stabilize, and watch the gauges for one minute without touching the brake. A small drop (a few psi) is acceptable; a larger or continuing drop is a leak. Then hold the brake pedal firmly applied for one minute and watch again - applied leaks are common at chambers and hoses.
Low pressure warning and spring brake test
Pump the brakes (engine off, parking brake released where safe) and watch for the low pressure warning to activate, typically around 60 psi. Continue pumping and confirm the spring brakes apply automatically as pressure falls further. This proves the fail-safe function works.
Governor cut-in / cut-out check
Note the pressure at which the compressor cuts out and cuts back in. If cut-out is too high, the safety valve may vent; if too low, you may not have full braking force.
Traps
- Confusing the one-minute leak test limits with the low pressure warning threshold.
- Forgetting to release the parking brake before the spring brake test (the test cannot work if the brakes are already set).
- Assuming a slow build is normal in cold weather; it may be, but it still must meet the manufacturer's limit.
Rule of thumb
"Build, hold, warn, set." Build pressure, hold it for the leak test, pump until the warning sounds, and confirm the spring brakes set.