Most all semi-truck models come equipped with an air brake system. The air brake system consists of the brake chambers, slack adjusters, scams, brake shoes and the brake drums. When the air is applied to the brakes, the brake shoes are compressed to the inside of the brake drums. When the air is released from the brakes, the brake shoes will retract to the proper distance from the inside surface of the brake drums. Over time, the brake shoes will wear down. Change the brake shoes before the shoe thickness reaches one eighth to one quarter of an inch.
Pull the semi-truck onto a flat surface and apply the air brakes by pulling out on the air brake knob. Turn the engine off.
Place the wheel chocks around the front and back of both front wheels to prevent the semi-truck from rolling when the rear end is jacked up. Move back to the inside of the truck and release the air to the brakes by pushing the air brake knob back in.
Jack the rear end of the semi-truck up with a hydraulic floor jack. Place the jack stands under the rear axle about 12 to 16 inches from the back side of the rear driver side wheel hub assembly and the rear passenger side wheel hub assembly. This will allow the weight of the truck rear end to be equally distributed onto both jack stands. Lower the rear end onto the jack stands and leave the jack in the upright position as a extra safety precaution.
Loosen and remove the lug nuts from both rear tandem wheels with the air wrench and a socket. Pull the wheels off of the drums by using a pry bar to pry the wheel back and forth as you are pulling out on the wheel at the same time. Lay the wheels flat down near each work area.
Slide under the rear of the semi-truck and locate the slack adjusters on both rear axles. Two slack adjusters are on the driver side of the rear axles, and two slack adjusters are on the passenger side of the rear axles. Turn the adjustment bolt on the back of the slack adjuster counterclockwise with a ratchet and a socket until the adjustment bolt stops turning. Once the adjustment bolt stops turning, this will mean that the brake shoes are completely free of the brake drum. Move to the other three slack adjusters and repeat the same process for retracting the brake shoes from the brake drums. Begin the brake shoe replacement process on the rear driver side axle.
Pull the brake drum off of the rear driver side axle with your hands. If the drum is stuck, use the hand-held sledge hammer to tap the rear of the brake drum until it is loose. Pull the brake drum completely off of the brake shoes by grasping the drum with both hands. Lay the drum on the surface near the working area.
Remove the top return springs and the bottom hold-down spring from each brake shoe with a brake spring removal tool. Pull the brake pads off of the hub assembly backing plate. Tap the spring retainers that are located about halfway down the side of each brake shoe, with the sledge hammer until the retainers are out of the brake shoes. Tap the new retainers into the sides of both new brake shoes with the sledge hammer until the retainers are completely inside of the shoes.
Slide the top return springs onto the top of the new brake shoes with the spring removal tool. Position the new brake shoes onto the backing plate. Make sure that the bottom of each brake shoe is positioned against the end of the scam. Slide the hold-down spring to the retainer clips inside of each brake shoe with the spring removal tool. Slide the brake drum over the new brake shoes until the front of the brake drum is flush with the brake shoes. Slide the wheel back onto the drum by using the pry bar to pry the tire up onto the drum. Screw the lug nuts onto the stud. Tighten the lug nuts with the air wrench and the socket.
Repeat the same exact steps as outlined above to replace the brake shoes on the other three rear wheels. Finish replacing the brake shoes on the rear axle before moving to the secondary rear axle. Once all of the brake shoes have been replaced and the wheels have been tightened by the lug nuts with the air wrench, adjust the brake shoes to the proper distance from the inside of the brake drums.
Slide back under the back of the semi-truck and adjust the brake shoes to properly fit the brake drums on both rear axles. Turn the adjustment bolt on the back of each slack adjuster clockwise until the adjustment bolt stops turning. This will mean that the new brake shoes are fully compressed to the inside surface of the brake drums. Turn the adjustment bolt one quarter to one half of a turn counterclockwise to position the new brake shoes to the proper distance from the drum. Use a flashlight if necessary to watch the brake shoes through the back side of the wheel hub assembly. Move to the other three slack adjusters and repeat the same process.
Jack the rear of the semi-truck back up and remove the jack stands. Lower the truck to the ground and remove the jack. Crank the engine and wait for the air pressure to build back up to 120 psi. Then, reapply the air to the brakes by pulling out on the air brake knob.
Remove the wheel chocks from the front wheels and drive the semi-truck in a safe location to test the operation of the new brake shoes. Park the truck.
Make sure that all of the slack adjusters are adjusted the same so that the rear brake shoes will work properly. Inspect the brake drums for any damage such as excessive wear or cracking. If the damage is excessive, replace the brake drum with a new drum.
If the brake shoes are not adjusted properly, it can cause the rear brakes to not work properly. When adjusting the brakes, be sure to only turn the slack adjuster adjustment bolt one quarter to one half of a turn counterclockwise to position the new brake shoes to the proper distance from the brake drums. If the brake shoes are positioned too far from the brake drums, the shoes will not be able to touch the drums. This will result in the brakes not working at all.