Braking Systems, Air Brakes and Vehicle Dynamics
What the exam tests
This domain tests air-brake operation, daily brake tests, low-pressure warnings, spring brakes, ABS and skid control. Expect questions on descending hills, brake fade and jackknife prevention.
Core rules
- Typical air systems build to about 8.3 bar, cut in near 7 bar and warn below about 4.5 bar; confirm the values in your vehicle manual.
- Test pressure build-up, leakage and the low-pressure warning before driving.
- Spring brakes apply automatically when air pressure falls too low; they are not a parking brake you can ignore.
- On a long descent, use engine braking and a retarder; do not fan or ride the service brakes.
- If ABS activates, keep steady brake pressure and steer; do not pump the pedal.
Worked example
During a pre-trip test, the air pressure falls below 4.5 bar and the warning buzzer sounds. The driver should stop, keep the engine running to build pressure and check for leaks. If pressure does not recover, the vehicle must not be driven. The reasoning: below the warning threshold, spring brakes may apply unexpectedly and normal service braking is degraded.
Traps
Do not ignore a slow pressure loss; it can become a failure on the road. Do not use the hand valve as a normal service brake. Do not continue downhill after the low-pressure warning to reach a workshop.