4.1 Spring Parking Brakes, the Park Control Valve, Compounding, and Caging

Spring Parking Brakes, the Park Control Valve, Compounding, and Caging

Quick Answer: A spring parking brake is a spring chamber mounted on the back of the service chamber. A powerful spring applies the same foundation brake mechanically, and air pressure holds the spring back (releases it). The yellow park control valve is held in by air. Pulling it exhausts the air and applies the springs, and it pops out by itself, typically at 20–45 PSI (138–310 kPa). As pressure falls, the springs start to expand at about 82–84 PSI, drag at about 55–60 PSI, and are firmly applied by about 45–50 PSI. Caging bolts release the springs mechanically for towing or repair only.

The idea: air releases, springs apply

The service brakes in Chapter 3 need air pressure to apply. Spring parking brakes work the other way round. The park component rides on the back of the service chamber and applies the regular foundation brake components with mechanical spring force instead of air. Air pressure from the service tanks is used to compress the spring and release the brake.

This design makes the park brake fail-safe:

• A parked vehicle stays held even if all its air leaks away overnight.
• If the air system loses pressure while driving, the springs apply automatically.

The park circuit is drawn in orange in the Alberta schematics. It takes its air from the higher of the primary or secondary service tank, so a failure in one service circuit does not apply the spring brakes. The driver keeps the use of the vehicle long enough to make a safe, controlled stop.

The spring brake chamber

The spring is compressed with enough force to be dangerous. The course uses discharged springs and cut-away chambers to show how far the spring is compressed. Never try to take a spring chamber apart. Corrosion on a chamber or its clamp ring can let the spring escape with tremendous force, so it must be repaired right away by certified heavy equipment technicians.

The park control valve (yellow button)

• Push in: delivers air to the spring chambers, releasing the park brakes.
• Pull out: exhausts the park air, so the springs expand and apply the brakes.
• Automatic pop-out: the yellow button sits on a spring-loaded spool that air pressure holds in. When pressure drops, the spring under the spool pops the button out. This typically happens between 20 and 45 PSI (138–310 kPa), although on new trucks it can happen as low as 10 PSI (69 kPa). A vehicle must be placed out of service if the park control does not default back to the park position when the air is depleted.

Two exceptions to know:

• Urban transit buses: the park control works the opposite way to the method described above.
• Some highway motor coaches: these have an isolated park tank. When that tank is depleted, the park control valve pops out. Its pressure is not shown on any dash gauge.

When the springs actually apply

The pop-out point is not the point at which the brakes start to grab. The Alberta course gives this typical sequence as tank pressure falls:

This is why the course advises that, in a park circuit failure, the driver should apply a steady service application and expect the spring brakes to begin applying gradually as pressure falls to about 55–60 PSI.

How strong is a spring application? Spring park brake applications are only about half as strong as a full service application. Whether the wheels lock in an emergency depends on the vehicle's weight, how sharply the brakes are applied (a shock application or a gradual one), and the available traction. Once the springs apply there is no control of the vehicle if it starts to slide. ABS operates only on the service circuits, so it will not help.

Compounding

Compounding means applying the service brakes and the spring brakes on the same chamber at the same time, so the two forces add together on the pushrod, slack adjuster, and foundation brake. Suppose a full service application produces a force F. A spring application is roughly F/2, so a full service application added to an applied spring would load the parts to about 1.5 × F. That is more than they are designed to take repeatedly.

Many vehicles are plumbed to prevent this. When a service application is made while the spring brakes are applied, some of that service air is sent into the spring section, which backs the spring off as the service force comes on. Even so, avoid a hard service application while the park brakes are set, unless a procedure calls for it. Remember too that pushing the pedal while parked with the springs applied is not the same as checking brake adjustment. Section 6.3 explains the correct method.

Caging bolts (release tools)

Caging bolts are used to pull the park spring off the application so that the vehicle can be towed or repaired. Rules from the course:

1. Caging bolts are not a way to take park chambers apart.
2. If all of the park brakes are to be caged, place chock blocks on both sides of the wheels first. Once caged, those wheels have no parking brake.
3. There is no rule about where caging bolts are stored, but they are typically in pockets on the side of the spring brake chamber, and some chambers build them into the body.
4. Do not continue to drive a vehicle whose park springs have been caged. Caging is for maintenance and towing only.