3.1 Spring Parking Brakes

The Core Concept of Mechanical Parking Brakes

Air brake systems operate differently from typical light-duty passenger vehicle hydraulic brakes. In a standard hydraulic brake system, pressing the brake pedal pushes fluid through lines to press the brake shoes or pads against the drums or discs. When the vehicle is parked, a mechanical cable system (usually connected to a hand lever or foot pedal) pulls the rear brakes into place.

For commercial vehicles equipped with air brakes, a different solution is required because of the properties of compressed air. Air pressure is used to apply the service brakes during normal driving operations. However, a parked vehicle with its engine off does not have a running compressor to replenish air. Because pneumatic systems are susceptible to slow leaks, relying on air pressure to hold a vehicle parked is unsafe. If the air leaked out over time, the parking brakes would eventually release, and the vehicle would roll away.

To resolve this safety concern, commercial vehicles are equipped with spring parking brakes. Spring parking brakes are designed to apply when air pressure is removed, and they require air pressure to be released. This is a fail-safe design: if a vehicle experiences a major air leak, the brakes will automatically apply to stop the vehicle, rather than releasing and allowing the vehicle to roll. The system uses a powerful mechanical spring inside the brake chamber to apply force to the brake linkage when there is no air pressure.

Dual-Chamber Actuators

To understand how spring parking brakes work, one must examine the brake chambers mounted on the vehicle's axles. Front steering axles are usually equipped with single-chamber service brake actuators, which do not contain parking springs. If front wheels were locked by a spring parking brake, the driver would lose steering control. Rear drive axles and trailer axles are typically equipped with dual-chamber actuators, commonly referred to as spring brake chambers or Type 30/30 chambers.

A dual-chamber actuator consists of two distinct chambers welded or clamped together in tandem:

1. The Service Chamber (Front): This chamber is located closest to the pushrod and slack adjuster. It contains a flexible rubber diaphragm and a light return spring. When the driver presses the foot brake pedal, compressed air enters this chamber, pushes the diaphragm and pushrod forward, and applies the brakes. When the pedal is released, the air is exhausted, and the return spring pulls the pushrod back to its original position.
2. The Spring Chamber (Rear): This chamber is mounted behind the service chamber. It contains a very strong, heavy coil spring (often capable of exerting between 1,500 and 2,000 pounds of mechanical force) and a separate diaphragm.

During normal driving, the spring chamber is supplied with air pressure from the parking brake circuit. This air pressure pushes against the diaphragm, compressing the powerful coil spring and holding it back in the released position. When the driver pulls the parking brake valve on the dashboard, or if there is a severe air leak in the system, the air pressure holding the spring compressed is exhausted to the atmosphere. This allows the heavy coil spring to expand. The spring pushes against a pushrod that extends through the service chamber, mechanically applying the brakes.

Pressure Thresholds and Automatic Application

Because the spring parking brakes are held in the released position by air pressure, their operation is directly tied to the system's air pressure. There are three key pressure thresholds that every commercial driver must memorize for the ICBC knowledge test:

The Yellow Push-Pull Dash Control Valve

The driver manually controls the spring parking brakes using the parking brake control valve on the dashboard. This valve is easily identifiable by its bright yellow color and unique diamond shape.

• Pushing the Yellow Knob In: Pushing the knob in supplies compressed air from the reservoirs to the spring chambers. This air pressure compresses the powerful springs, releasing the parking brakes and allowing the vehicle to move.
• Pulling the Yellow Knob Out: Pulling the knob out cuts off the supply of air and vents the air inside the spring chambers to the atmosphere. With no air pressure to compress the springs, they expand and mechanically apply the parking brakes.
• Automatic Pop-Out Feature: The yellow valve contains a pressure-sensitive spring mechanism. If the system air pressure falls to the 20 to 45 psi range, the valve will automatically "pop out" to the pulled position. This vents any remaining air in the spring chambers and ensures the parking brakes are fully applied.

Mechanical Releasing (Caging the Brakes)

If a vehicle suffers a complete failure of the air system (e.g., the engine will not start, or the compressor is broken) and needs to be towed or serviced, the wheels will be locked because there is no air pressure to release the spring brakes. In this situation, the brakes must be released mechanically. This process is known as caging the brakes.

To cage the brakes, the driver must use a caging bolt (also called a T-tool), which is typically stored in a bracket on the outside of the brake chamber or in the tool box.

1. Remove the Dust Cap: The driver removes the rubber dust cap from the center hole at the back of the spring chamber.
2. Insert the Bolt: The caging bolt is inserted into the hole. The T-shaped end must pass through an opening in the internal pressure plate.
3. Lock the Bolt: The driver rotates the bolt a quarter-turn (90 degrees) to lock the T-head into the internal pressure plate.
4. Tighten the Nut: A washer and nut are threaded onto the outer end of the bolt. Using a wrench, the driver tightens the nut. This action pulls the caging bolt outward, which mechanically pulls the internal pressure plate back, compressing the heavy coil spring and releasing the brakes.

[!WARNING] Block the Wheels Before Caging: Before performing any caging procedure, you must always block the wheels with wheel chocks. Caging the spring brakes completely removes the vehicle's mechanical parking brake capability. If the vehicle is parked on a grade and the wheels are not blocked, it will roll away as soon as the springs are caged, which can cause severe accidents or fatalities.

[!CAUTION] Never Disassemble the Spring Chamber: The spring chamber is held together by a heavy metal clamp. The internal coil spring is under extreme mechanical tension (up to 2,000 lbs). Never attempt to loosen or remove the clamp on a spring chamber. Doing so without specialized factory tools will cause the spring to release explosively, which can cause severe injury or death.