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 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 disipates allowing the spring to extend the pushrod which in turn applies the brake.