The service brake chamber is used when only the application of service brake air is required on an axle. There will be no emergency braking provided on this axle. You will find
service brake chambers on steer axles, some rear axles, and old trailers that use a relay emergency valve to apply parking air pressure to the service chamber.
Spring Brake Piggybacks
The spring brake Piggyback is a replacement part for repair
of a spring brake chamber. There is a heavy spring inside that applies the mechanical force needed for the parking
and emergency brake functions. Service of the piggyback should never be attempted because the large spring inside can be compressed with a force up to 3,000 lbs. Severe injury or death can easily result should the spring be accidentally released.
Spring Brake Assembly
The spring brake assembly consists of the service brake and parking/emergency brake. The service brake is the section at the clamp closest to the axle. The diaphragm, pushrod and return spring may be serviced by removing the clamp. The spring/emergency brake is everything from the first clamp to the end farthest from the axle. The spring brake must be completely replaced if it is faulty.
How Spring Brakes Function
The spring brake assembly is bolted to a mounting bracket on the axle. The spring brake pushrod clevis is attached to a slack adjuster which is attached to a camshaft. As the spring brake pushrod is extended from the chamber, either by the parking/emergency brake or by service air pressure, the linear force of the pushrod is converted to rotational force by the slack adjuster and camshaft. This spreads the brake shoes against the brake drum.
When air is applied to the parking/emergency brake, the load of the heavy spring is released allowing the wheels to turn. When the driver steps on the brake pedal, a signal is sent to a relay valve that then sends air pressure to the service chamber of the spring brake. The air pressure causes the pushrod to extend which in turn applies the brake. This air pressure is released once the driver takes his or her foot off of the brake pedal.
The spring brake is designed to apply the brakes automatically in the event that there is a loss of system air pressure. This happens because the air pressure holding back the heavy parking/emergency spring dissipates allowing the spring to extend the pushrod which in turn applies the brake.