Spring Brakes are normally found on the rear axle of heavy trucks and are used as the parking brake. The brakes on heavy trucks differ from passenger cars, because they use air pressure instead of hydraulic pressure to apply the braking power. When you need to slow down your car, you press the brake pedal forcing brake fluid, from the reservoir, through the master cylinder, where it is assisted by the power braking system. The fluid then passes through the ABS control module, proportioning valves and on to the calipers. Once the fluid reaches the calipers, the pressure squeezes the pads against the rotors causing the car to slow down. This same principle is used in heavy trucks except the hydraulic pressure is replaced by air pressure.
Air brakes in heavy trucks work in two ways. When air pressure is applied to the cylinder it squeezes the pads against the rotor in the same way as a passenger car, with the main difference being the addition of the spring brakes. Most passenger cars use a cable actuated parking brake system that can be set or released using a handle located in the center console, or a pedal located to the left of the brake or clutch. The parking brake on an air system is actuated by the spring brake. The spring brake works in two different ways. First pressure is applied to the spring side allowing the parking brake to disengage. The amount of pressure required varies but the spring is fully decompressed at 20 lbs. Once the spring is fully decompressed, the parking brakes have been applied.
This type of braking system was started as a safety measure. Before the use of spring brakes, a truck that had lost air pressure, due to a system or hose failure, would have no way of coming to a controlled stop. Spring brakes help to alleviate this problem. As the air pressure drops, the spring decompresses allowing the driver the time to come to a controlled stop. If the pressure ever drops below 60 lbs, an alarm will sound alerting the driver to a potential problem. This gives the driver time to come to a safe and controlled stop.
Spring brakes have two air chambers in them, one to release the spring, and the other to apply braking power. Once the spring has been compressed, the parking brakes have been released. The spring brakes then work by applying pressure to the service brake chamber to slow down the vehicle. If there is an instantaneous loss of pressure, the spring is strong enough to apply full braking power to the axle, which it is mounted on. This doesn’t provide the same braking power as all the service brakes working together, but it is enough to keep the vehicle under control.
If the vehicle needs to be moved and there is no air pressure to release the springs, a cage bolt is required. All spring brakes come with a way to mechanically release the spring tension. A bolt is locked into place inside the chamber, and a nut is turned until the spring pressure is released, thus allowing the vehicle to be moved. Once the vehicle has been moved to the desired location the bolt is removed and the parking brake is reset. The bolt should never be left in the brake cylinder because the vehicle could move unexpectedly.
While this is just a brief overview of the differences between spring brakes and passenger car brakes, it should help you to understand them better. Remember, spring brakes are actuated by air pressure, contain a large spring that acts as the parking brake, and can be mechanically actuated in case of emergencies. Any service to the spring brakes must be left to trained professionals because of the immense pressure applied by the spring