Air Supply, Compressors and Reservoirs
Where the air comes from and where it goes
The compressor is engine-driven and engine-lubricated, so it must have a sound drive belt or gear and clean oil. The governor senses reservoir pressure and lets the compressor pump until cut-out (commonly about 120 to 125 psi), then unloads it until pressure falls to cut-in (commonly about 100 psi). The safety valve, usually set near 150 psi, protects the system if the governor fails. Air then passes through the air dryer (desiccant, purge valve, heater) into the wet tank, which is the first place moisture and oil collect, then through one-way check valves into the primary and secondary dry tanks. Check valves matter because they stop a failed circuit from draining the healthy ones.
Traps
Thinking the safety valve regulates normal pressure - it is a last-resort relief device. Draining only the wet tank and forgetting the others. Believing an air dryer removes the need to drain tanks daily. Assuming a compressor that runs constantly is normal; a healthy system unloads. Forgetting that reservoir capacity must allow several full brake applications with the engine off.
Rule of thumb with a worked example
The spread between cut-out and cut-in is normally about 20 to 25 psi. On the build-up test, with the engine at fast idle, a system that climbs from 85 to 100 psi in roughly 45 seconds is well inside a typical three-minute limit; one that takes six minutes is failing and the vehicle should not go on the road. Confirm exact build-up and leak limits in your provincial manual before testing.
Why it matters
On a long downgrade you may make many applications. If the compressor cannot keep up with leakage, pressure keeps falling and the spring brakes will eventually apply in traffic.