What is the acceptable air pressure loss rate during the static leak test with spring brakes released?

THE STATIC LEAK TEST evaluates the integrity of the entire air brake system — lines, fittings, chambers, valves — by measuring how much pressure is lost when the system is pressurized and static (no movement, no brake application). PROCEDURE: (1) Build air pressure to governor cut-out range (120-125 psi); (2) Turn engine off; (3) Release spring brakes (pull out knob — brakes off); (4) Wait for system to stabilize; (5) Time for 1 full minute; (6) Record pressure drop. ACCEPTABLE LIMITS: SINGLE VEHICLE (straight truck, bus): less than 3 psi in 1 minute. COMBINATION VEHICLE (tractor-trailer): less than 4 psi in 1 minute. TRACTOR-SEMI plus FULL TRAILER (B-train or triple): less than 6 psi in 1 minute. FAILING THE TEST: If the vehicle loses more than the allowable amount, there is a significant air leak somewhere in the system that must be found and repaired before operating. Common sources of leaks: loose fittings (often audible as hissing); cracked or perished hoses; failing valve seals; leaking brake chamber diaphragms. SECOND TEST: After the leak test, depress and hold the foot brake and conduct a second 1-minute test. Acceptable loss: less than 4 psi (single) or 5 psi (combination). The additional loss during brake application indicates the condition of the service side of the brake chambers.