If a vehicle's air brakes feel spongy, or do not offer adequate stopping power, they might need the be adjusted. It's possible to remove the slack in your air brakes with a few tools you probably have in your garage. This technique only works for manual slack adjusters, however. Newer brake systems feature automatic slack adjustment.
Apply chocks to the wheels so the vehicle won't roll without brakes. Ensure the air in the system is at full pressure. Activate the emergency brake, and press the relief valve in the system to bleed all of the air out.
Test the push rod. Mark the location of where the bottom of the push rod exits the air chamber, using a piece of chalk. Using a metal rod, pull out the push rod from the brake chamber as far as it will go. Measure the distance from your first chalk mark, to the bottom of the push rod, where it exits the brake chamber. This is your stroke distance. Anything over 3/4 of an inch requires slack adjustment.
Adjust the manual slack adjusters. Locate the adjusters close to the push rod. If the push rod is leaving the chamber, you are turning the adjuster the wrong way. Tighten the adjuster in the correct direction until you feel a slight resistance. The brake linings should meet with the brake drum. Unscrew the adjuster 1/4 to 1/2 a turn to allow the brakes to keep from touching the linings.
Re-check the push rod. Measure the rod's stroke distance one more time. If the distance is less than 3/4 of an inch, you have correctly removed the slack from your air brakes.
