All vehicles with air brakes must have a way of stopping if the service brake system fails. Most vehicle manufacturers combine this emergency braking system with a parking brake system using spring brakes.
Spring brakes are not air applied like service brakes. They apply when air pressure leaves the brake chamber and release when air pressure builds up in the chamber.
Spring brakes use a different type of brake chamber from service brakes. A brake chamber that includes both service brake and spring brake sections is called a spring brake chamber. (See Diagram 4-1.) Spring brake chambers apply the brakes by means of a large coil spring that provides enough force to hold the brakes in the applied position, instead of using air to apply the brakes.
Spring brake chambers are different in appearance from service brake chambers. To accommodate the large coil spring, a section must be added to the service brake chamber that is clearly visible and adds significantly to its size. The spring brake section is ‘piggy-backed’ onto the service brak e section and these two sections function as two separate chambers. The portion nearest the pushrod end is the service brake section and it works in the same manner as a separately mounted service brake chamber.
To release the spring brakes, normally about 414 kPa (60 psi) of air pressure must be supplied to the spring brake chamber to compress or ‘cage’ the spring. If system pressure is below 414 kPa (60 psi), the spring brakes start applying because there is no longer enough pressure to keep them released. (Note that this number may depend on the age of the brakes, newer brakes may require 90–95 psi before they release, and older brakes may take as little as 35 psi to release.)
Many vehicles can still be driven even with the spring brakes applied because the spring brakes do not have the braking power of the full service brake application. Before driving the vehicle, it is important to ensure that the air brake system has enough air pressure (normally 414 kPa (60 psi)) to keep the spring brakes from applying. Due to the way most spring brake chambers are currently constructed, it is very difficult to unintentionally release the spring.
The large coil spring used in the spring brake chamber is compressed under very high tension. Tampering, damage or corrosion can cause the spring to release, resulting in sudden violent motion of parts of the air brake chamber. Since this can be hazardous, never attempt to service or repair any air brake chamber.