A spring-brake chamber functions as a service-brake chamber, an emergency brake in case of air-pressure loss somewhere in the system, and as a reliable spring-applied parking brake (Fig. 33). Spring brakes are installed in the same manner as service brakes and are always installed on the front tandem axle. Spring brakes are often installed on both rear axles in a tandem-axle unit. They are a reliable parking brake because they are held on by spring pressure and require no air.
Spring brakes consist of two separate air chambers. The front chamber is essentially a service-brake chamber, and is used to perform the service-brake function. The rear chamber houses a large, powerful compression spring and diaphragm and performs emergency and parking functions. It is sometimes called a “piggyback.”
CAUTION: Never disassemble a spring brake. Serious injury may result. All discarded spring-brake chambers must be disassembled and disposed of by a trained professional.