Parking Brake Systems, Wheel Bearings, Seals and Related Components
Parking Brake Systems
Spring brakes
Most heavy trucks use spring brake chambers: a powerful spring applies the brake, and air pressure compresses it to release. Loss of air therefore applies the parking brake - the fail-safe principle. The spring brake needs adequate pressure to fully release (check the chamber specification; full release is typically near 60 psi), so low system pressure causes drag, heat, and burned linings.
Controls
- The yellow diamond push-pull valve is the parking brake control; on many vehicles it pops out automatically at low pressure.
- A relay valve speeds air flow to the spring chambers.
- The anti-compounding valve keeps service brake air from adding to spring force. Without it, a service application while parked compounds force and can damage the chamber and foundation brake.
- An inversion valve is used in some systems to apply spring brakes when the control signal is lost.
- DD3 safety actuators use two diaphragms and no spring; they apply on loss of air and are serviced differently - never assume the caging procedure matches a spring brake.
Safety
Cage (wind off) a spring brake with the release bolt before removing it, and follow the manufacturer's procedure exactly - the spring stores lethal energy. Caging is for temporary release only, never a repair.
Testing
With the parking brake applied, the vehicle should not move at idle in gear on a grade, and the pushrods should stay applied. Verify the park valve releases promptly and the chambers fully release without dragging.
Worked example: a driver reports the tractor will not roll with the park valve pushed in - confirm system pressure is above the release minimum, then check the park valve, relay valve, and for a stuck inversion valve.
Rule of thumb: if the spring chambers never fully release, suspect pressure and control valving, not the spring itself.
Wheel Bearings, Seals and Related Components
Oil-bath vs. grease
Most line-haul drive and trailer axles use oil-bath hubs: gear oil (commonly 75W-90 or 80W-90, or an OEM-specified synthetic) lubricates the bearings and carries heat away. Check the oil level at the fill hole with the wheel level - oil should be at the bottom of the hole. Grease-packed hubs are common on steer axles and need repacking at service intervals.
Adjustment
Seat the bearings while rotating the hub, torque the adjusting nut to specification, then back off and set end play - commonly 0.001 to 0.005 in measured with a dial indicator, or a specified preload on some designs. Too loose causes wheel wobble, seal leakage, and uneven tire wear; too tight causes heat, bluing, and rapid failure. Install the lock ring and nut per the axle maker's procedure.
Seals
The hub seal keeps oil in and water and dirt out. Always replace the seal when the hub comes off; drive it squarely with the correct driver; never reuse one. Inspect the spindle seal journal for a groove - a worn journal leaks even with a new seal.
Inspection and diagnosis
Look for spalling, pitting, bluing, and roller end wear. Oil on the brake shoes or a wet inner dual usually points to the inner seal; a wet hub cap can mean a bad gasket, loose plug, or missing vent. Overfilling makes seals weep.
Worked example: a trailer hub shows oil on the inside of the tire and the brake shoes are saturated - replace the inner seal and the contaminated shoes; oil-soaked linings cannot be cleaned and must be replaced.
Rule of thumb: if you can feel bearing play by hand, it is too loose; if the hub is hot enough to sizzle water, it is too tight.