How to Read Your Brakes
Solutions to Abnormal Wear & Performance Issues
Brake-caused accidents and inspections with violations are both largely preventable by effective troubleshooting in three areas:
- Air Pressure System - D.O.T. looks for leaks anywhere in the system. You should too.
- Foundation Brake - Having the push rod stroke out of adjustment is the single biggest reason for D.O.T. citing faulty brakes.
- Brake Lining - It can't be worn below the recommended service level or worn unevenly to any great degree.
Fade, poor stopping and brake lockup are problems that could be caused by air system malfunctions, wrong friction or bad brake maintenance. A drum that is heat checked can cause fade and poor performance during a panic stop.
Ways to prevent heat checking:• Replace all hardware at the time of reline• Use the friction material recommended for the application• Make all adjustments equal: what you do to one wheel, do to all other wheels• Make sure drum is in good condition
Relined Block Delaminations
An old shoe that is stretched can cause the lining to crack or separate Check old shoes for elongated rivet holes. This is a sign that the brake lining is "walking" on the shoe, and eventually will shear off the shoe table.
Check all shoes with a stretch gauge to determine if they're in spec. Check shoes for worn cam roller seats, which can be a cause of uneven wear.
Uneven Wear of Edges or Ends
End wear indicates a drum with bell mouth condition or worn bushings indicates bent spider.
Check drum diameter for bell mouth condition and turn drums or replace. Replace cam bushings at every reline. If shoe ends or lining must be ground to allow shoes to fit, or for the drum to go on, you may have a bent spider or the wrong shoes.
Rate of Wear on Lining Differs
By law, all vehicles manufactured after June 1980 must have operable brakes on all axles of all units originally equipped with brakes. If each wheel has the same mechanical input, the same linings at proper frictions levels, and the air system is ok, there should be no noticable differences in rate.
Use a ruler to measure the push rod accurately. Measurement should be the same - applied and released. There should be no excessive play - the push rod must begin turning the cam head immediately. Both brakes across the axle and all brakes on a tandern set must have the same adjustment. The result of every brake receiving the same mechanical input is even wear and predictable performance. In other words, a balanced braking system.
Brake Adjustments Not Holding
If the slack adjuster angle is more or less than 90° at readjustable stroke length, the push rod can be the wrong length, which can cause frequent brake adjustments.
Check slack adjuster for internal wear. See how far it moves before the S-cam rotates. Push rod movement should relate to shoe movement against the drums.
Excessive Wear in Center
Excessive center wear on lining indicates friction level is insufficient for your application.
Replace worn-out lining with new lining having the proper friction level for your application.