Spring Breaks

Spring brakes are a separate portion of the air brake can, or piggyback. This portion is used mainly for parking the truck, although there is some assistance with the service brake function in full truck applications. If air to the primary supply (drive axles) is exhausted (blown air line), the SBM/Inversion valve takes over and releases a proportional about of spring brake air from the cans (depending on driver brake pedal input), energizing the brakes. This supply air comes from the secondary (steer) axle and would have approximately 4-5 brake applications. Again, this is not tractor functionality.

Also, air can be exhausted from the spring brakes by pulling the yellow knob in the dash which will lock the drive axle brakes.  Thus, the spring is released from its compressed driving position, pushing on the rod, and setting the brakes. In most cases you will see these piggyback cans only on the drive axles.  Some manufactures use only 2 spring brakes on a tandem drive axle configuration, only with a tractor, but never a straight truck.

FMVSS 121 sets guidelines for air capacity, stopping, and parking. The parking brake must be large enough to hold a fully loaded truck on a 20 percent grade with all liftable axles in the up position. The reason that the up position is important is to protect against air depletion during truck park, resulting in loss of lift axle ground contact. Some OEs install parking brakes on large capacity lift axles and view this test differently. Do not be concerned if your OE does not supply piggybacks, only service brakes, on the auxiliary axles, there is no safety preference here.

Rest assured that air volume is correct for your vehicle when it is built at the factory, regardless of the practice. The Super Dump in the picture above has met all requirements, IF, the sales order included data about the pushers, as well as the flying tag. If all of the liftable axles were installed in the aftermarket, and the intended service was not reflective of actual use, then guarantees about sufficient air volume are negated.

For this reason, when you find yourself adding any axles in the aftermarket, make certain that you document your intent on the order, or purchase ‘up-sized’ parking brakes. The evaluation considers how much more weight the vehicle will carry. For this reason, a single small pusher typically does not increase the need for increased spring brake size. If you add a large capacity (20K or more) or multiple pushers, air volume requirements increase, along with spring brake size.