Pressure Buildup, Leakage and Low-Pressure Warning Tests
Run the tests in a set order
Learn the sequence so a question about 'what comes next' is easy:
- Buildup test - engine at fast idle, watch the gauge rise through the normal range.
- Leakage test, brakes released - engine off, build to full pressure, note any fall over one minute.
- Leakage test, brakes applied - hold a full service brake application and watch for loss.
- Low-pressure warning test - pump down (fan the brakes) and confirm the buzzer/light comes on.
- Spring brake (emergency) application - keep pumping down and confirm the spring brakes apply.
- Governor check - confirm cut-in and cut-out occur at normal pressures.
What counts as a pass
Commonly quoted benchmarks are that the system builds from roughly 50 to 90 psi within about three minutes at fast idle (some standards phrase it as 85 to 100 psi within two minutes), and that leakage does not exceed about 2 psi per minute with brakes released or about 3 psi per minute with brakes applied. Use the figure printed in your province's standard, but remember the principle: a steady pressure loss is a defect, not a quirk.
The low-pressure warning must come on before pressure falls below about 60 psi, and the spring brakes should begin applying automatically somewhere in the 20–45 psi range as pressure continues to drop.
Traps
- Doing leakage tests with the engine running or the system not fully charged.
- Forgetting to chock the wheels - the vehicle can move when spring brakes release or apply.
- Confusing slow buildup (a supply fault) with leakage (a containment fault).
Rule of thumb: if you can hear air or the gauge keeps sliding, stop and fix it - you have found a real fault, not a test result to sign off.