Brake Systems and Components
Brake parts questions reward knowledge of system type, foundation brake, and air system components. Medium-heavy trucks use air brakes, hydraulic brakes, or a combination; each has distinct part families.
Concepts you must know
- Foundation brakes: drum (S-cam) and disc (air disc). Know shoes, linings, drums, rotors, calipers, and slack adjusters.
- Air system components: compressor, governor, air dryers, reservoirs, valves (relay, quick-release, proportioning), and spring brake chambers. Chambers are rated by stroke and size and must match the application.
- ABS and stability systems: wheel speed sensors, modulators, and ECUs; parts are application-specific.
- Hydraulic systems: master cylinder, calipers, and proportioning valves on lighter medium-duty trucks.
Traps in questions
- Mixing service chamber and spring brake (parking) chamber part numbers - they are not interchangeable.
- Assuming a lining or drum from one axle rating fits another; size and rating matter.
- Overlooking that air dryers have service intervals and desiccant cartridges are wear items.
- Confusing slack adjuster (manual vs. automatic) types.
Rule of thumb
Match brake parts by axle rating, brake size, and system type (air vs. hydraulic). When in doubt, confirm chamber size and stroke and whether the position is steer, drive, or trailer - the same truck can use different chambers at each position.