Air Brakes
Air Brakes
The Air Brakes test checks whether you understand the system that stops a Kansas commercial vehicle: compressor, governor, reservoirs, foot valve, slack adjusters, spring brakes, and ABS. It also tests the required inspections and the warning signs of a failing system.
Core rules
- Air pressure is built by the compressor and controlled by the governor, which cuts out near 125 psi and cuts in near 100 psi. Drain air tanks daily to remove moisture; in Kansas winter weather, frozen lines can disable brakes.
- The low-pressure warning must activate before pressure falls below 60 psi. Spring brakes apply mechanically when air pressure drops to roughly 20–45 psi, so a loss of air is a stopping event, not a warning to ignore.
- Before driving, perform the air brake tests: build to governor cut-out, shut off the engine, release the parking brake, apply the service brake, and hold for one minute. Leakage should not exceed 3 psi for a single vehicle or 4 psi for a combination.
- Check slack adjusters and push rods; excessive travel means the brakes need adjustment. Do not fan or ride the brakes on long downgrades; use a lower gear and controlled braking.
Worked example
A tractor-trailer builds to 120 psi. With the engine off, parking brake released, and service brake applied, the gauge reads 120 psi at the start and 115 psi after one minute. That is a 5 psi drop. The combination limit is 4 psi per minute, so the vehicle fails the static leak test. The driver should not dispatch it; a leak in a line, chamber, or valve must be repaired before the vehicle is driven.
Traps
ABS does not shorten stopping distance on all surfaces and does not replace safe following distance. A warning buzzer that stops does not mean the problem is fixed. Do not test for leaks with the engine running or the parking brake applied and call it a complete test.