In a pneumatic (compressed air) braking system of a heavy vehicle, the parking brake works using a spring brake mechanism — this is why it’s often called a spring-applied, air-released brake.

In this article, we provide a detailed explanation of working principle of parking brake in pneumatic system.

Function of Parking Brake (Handbrake):

The parking brake is used to keep a vehicle stationary when parked. It mechanically locks the wheels (usually the rear wheels) to prevent the vehicle from rolling, even on slopes, and can also serve as an emergency brake if the main braking system fails.

Working Principle of Parking Brake in Pneumatic System

Spring Force Applies the Brake (Default State):
- Each brake chamber on a heavy vehicle has two sections:
  - Service brake section (for normal braking)
  - Spring brake section (for parking/emergency brake)
- Inside the spring brake section is a large, powerful coil spring that pushes on a diaphragm or piston, applying the brakes mechanically.
Compressed Air Releases the Brake (Normal Driving):
- When the air system is charged, compressed air pushes against the spring, compressing it and releasing the brakes so the vehicle can move.
Parking Brake Applied by Releasing Air:
- When the driver pulls the parking brake knob, air is vented out of the spring brake chambers.
- The spring is no longer compressed and applies force to the brake linkage, keeping the brakes engaged (wheels locked).
Fail-Safe Function:
- If air pressure in the system drops below a safe level (due to leakage or compressor failure), the springs automatically apply the brakes — this acts as an emergency brake.

Key Advantages:

Fail-safe design: Brakes apply automatically if air pressure is lost.
No separate mechanism required: Parking brake uses the same system as service brake but with a mechanical spring.
Holds vehicle securely: Even if air leaks overnight, the spring keeps the brake applied.