Vehicle Systems, Brakes, and Inspection Failures
Vehicle Systems, Brakes, and Inspection Failures
This domain tests how the brake system works, how it fails, and which defects take a bus out of service on the spot.
Air brake fundamentals
A dual-circuit air system stores air in tanks and applies the service brakes through the treadle valve. The compressor governor cuts out near 120 to 125 psi and cuts back in around 100 psi. The low-pressure warning must activate before pressure falls below 60 psi, and the spring brakes begin to apply as pressure drops further, typically around 45 psi on many systems. Know three systems: service, parking, and emergency.
The tests that matter
- Static leak: engine off, brakes applied, no more than 3 psi loss in one minute for a single vehicle, 4 psi for a combination.
- Applied leak: hold the pedal, no more than 3 psi per minute.
- Build-up: 85 to 100 psi within 45 seconds at fast idle.
- Slack adjusters: pull each by hand; free play beyond about one inch means out of adjustment.
- The yellow ABS lamp staying on means ABS is disabled, though normal braking remains.
Hydraulic systems get a firm pedal that does not fade under steady pressure, a clean fluid level, and no warning lamp.
The eight-way light system, stop arm, and emergency exits are vehicle systems too. A stop arm that will not extend is not a cosmetic defect - it is a legal failure of the bus.
Worked example
The low-air warning does not sound until the gauge reads 40 psi. The standard is 60 psi, so the bus cannot carry students until it is repaired, and the driver writes it up rather than driving it.
Traps
Mixing up the static and applied leak tests; forgetting to test build-up; assuming an ABS lamp means brakes are gone; ignoring slack adjuster travel.