Air brakes: why losing air stops the truck

Run the CDL pre-trip air-brake checks, one step at a time. Press the glowing step.

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PSI 0 LOW AIR BZZZ! PARK POP! tok COMPRESSOR chug chug AIR TANK psssh CHAMBER INSIDE THE BRAKE CHAMBER air side spring drum BRAKES OFF Compressor fills the tank. Tank air pushes the spring open. Spring open = brakes off.
The big idea: air holds the brakes OFF. Lose the air, and springs slam them ON.

Numbers follow the federal CDL air-brake checks: compressor builds 85 to 100 psi in 45 s, governor cuts out near 125 psi, low-air warning before 60 psi, spring brakes apply between 20 and 45 psi (drawn at 35), leakage limits for a combination vehicle of 3 psi/min parked and 4 psi/min with the brake applied. Test speeds are sped up where the badge shows ×. Your state's CDL manual is the authority: check it before relying on any figure here.