While there are some short-cut methods to determine brake adjustment but they will not be accurate enough to pass a DOT inspection. It adjusts and takes up the slack in the stroke as the friction surfaces wear away. But since there are no such things as normal truck brakes, slack adjusters are designed to be adjusted.
You can release the spring brakes and build the system pressure to psi, make a full pressure application and note the pressure drop. According to some a 2-psi per axle drop in system pressure, or psi on a 5-axle unit, is acceptable. The problem is, how do you know that two brakes are adjusted well below the limit and three are over-stroking by an eighth of an inch? We will need to evaluate your personal credit to continue.
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Step 2: Click on By Measurement on the left menu bar. The A measurement. Take your tape measure and measure the arm length of the slack adjuster.
It requires you to measure from the center of the big gear at the bottom of the slack adjuster to the small pin at the end of the arm. Click the Arm Length box and select the measurement from the drop-down menu. The B Measurement. This is not really a measurement, rather the next step is to count the number of splines. The number of splines indicates whether it is coarse or fine. Click the Number of Splines box and make your selection from the drop-down menu. You will typically find 10, 24, 28, or 37 splines on an automatic slack adjuster.
Adjusting the slack adjuster returns the brake components to their proper tolerance and maintains optimal braking performance.
This procedure takes only a few minutes and minimal mechanical skill. Start the vehicle and bring the air pressure in the tanks to maximum. Once the air pressure reaches maximum, shut off the engine. Place wheel chocks securely on both sides of one wheel to prevent the vehicle from moving during the procedure. Push in the air valve on the dashboard of the truck, releasing the brakes to be checked and adjusted. Place a ground cloth or creeper on the ground or floor to keep from getting dirty while underneath the vehicle.
Air brakes can have different sized adjustment bolts, so know or find out what size of wrench is needed. Checking the free play on the pushrod. Using the chalk, mark the pushrod at the point where it enters the air cylinder. Pry back on the slack adjuster using a screwdriver or pliers to pull on it and move the slack adjuster back as far as it will go.
This will cause the pushrod to pull part way out of the air cylinder. Measure the distance between the air cylinder and the chalk mark. There is one pushrod for each brake assembly at each end of every axle so check each of them.
Locate the adjustment bolt, which is located on the base of the slack adjuster. Often there is a sleeve around the bolt that needs to be pushed inward to access the bolt. By using a box end wrench, the sleeve can be pushed down with the wrench.
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