Tech Support

Brake Shoe Identification

14 1/2″ Diameter

18″ Diameter

TBS Brake Lining Identification Worksheet

How to Read Your Brakes

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.

Faliure Analysis

Signs of Unusual Wear:

(DISCLAIMER: Before beginning, please note that not all brake complaints and problems can be solved by only brake shoe inspection. Common problems also occur as a result of high temperature issues, such as air, mechanical or friction imbalances. Any imbalance in one or a combination of all three will appear as a friction material issue. The necessity for inspecting and replacing all worn parts has become more critical with the advent of automatic slack adjusters and ABS.)

When brake drums are removed, the brake lining should be inspected for signs of unusual or atypical wear. Common problems may arise as a result of ignoring hardware upkeep and s-cam bushings. These problems include but are not limited to: Bottom brake shoe worn more than the top shoe; Lead shoe blocks primary contact, with very little contact on the anchor block; Excessively noisy brakes.

Know Your Brake Temperatures

Brake Temperatures

(Note: Temperatures indicated are at the lining/drum interface and are approximate)

550°F – 650°F – Brake resin odor is present
850°F – Brakes begin to smoke
1,100°F – Brake oxidation occurs at parts of the brake open to air rushing by
1,250°F – Drums become cherry-red internally
+1,250°F – Danger of run-away due to excess drum expansion

Brake lining will remain within its designated friction range on a properly maintained brake through all of the temperatures described under three conditions:

1. 1. That the rated axle capacity of the brake material has not been exceeded. If this condition is present, more than normal fade can be present at higher temperatures.
   2. That at temperatures above 850°F, the driver does not over-speed the vehicle where severe or sudden braking can be expected. Under safe operating conditions, the brake lining on a properly applied and maintained brake will remain stable at high temperatures. Excessive speed while the brakes are hot may result in longer stopping distances.
   3. That the brake lining is not contaminated with oil or grease.

Identifying Brake Noise

Possible Causes:

Brake noise is the result of the linings and drum/rotor interacting with a given variable in the friction forces between these elements. This creates axial vibrations of the drum/rotor and component parts.

The following is a list of the most common specific causes of brake noise:

1. 1. Poor lining-to-drum contact
   2. Imbalanced braking condition
   3. Type and condition of the suspension system
   4. Brake drum design and weight:
      Changing weight by machining on lathe
      Machining with pointed tool
   5. The condition of the brake component parts
   6. The brake design
   7. High friction materials used in light-duty applications
   8. Linings contaminated with oil
   9. Over-adjustment of component parts
      Linings dragging against broken surface of drums
   10. S-cams rotating reverse of wheel rotation
   11. Shoes and lining rubbing drum wall
       Return springs – broken, stretched or missing
       Worn anchor pin
       Worn anchor pin hole (shoe or spider)
       Bent spider
       Loose or bent camshaft
   12. Linings loose on shoe
   13. Camshaft bushings worn or need lubrication
   14. Flat spots on cams or rollers
   15. Brake chamber clevis pins worn or bent
   16. Push rod and slack adjuster not aligned
   17. Out of round drum
   18. Automatic slack adjusters not set up properly. (Note: All slack adjusters must be reset after each brake reline or new installation, following proper template. Haldex is the only slack which does not require a template because of its installation indicator on the slack)

Brake Maintenance

Best Practices to Keep Your Brakes in Good Condition:

Air Systems – Ensure your brake system is well-balanced.
Chambers – Make sure your push rod is even on all chambers. To help, cut the push rod to the appropriate length.
Slack Adjuster – First, set your thorn up properly. 90 degree rules for manual apply. Also, use templates for automatic slacks (Exception: Haldex).
S-Cams – Make sure no grease is on your S-cam head. Aim for free play movement in and out, and up and down. Also, seal your installation.
Hardware Kits – Replace these completely, not only the springs.
Shoes – Do a complete replacement, not only one wheel. The minimum should be a complete axle if it’s not possible to do an entire unit.
Drums – Make sure to use heavy drums for severe applications.
Lining Wear Patterns – To begin, after removing drums look at the brake shoes and lining before you remove them off the wheel – it will tell you a story.
Brake Adjustment – Jack up axles and snub up brakes, then back off ¼ to ½ turn on manuals and use templates on automatics.
Brake Lining Burnishings – Heat up the brakes until you are able to smell them. Then, allow them to cool. Do this procedure within ¼ to 1 mile. Check wheel temperature with a heat gun.

How To Reline Your Brakes

Suggested Brake Relining Procedures

1. 1. Remove the drum and inspect it for general wear, cracks, heat-checks and other evidence of potential over-heating.
   2. Inspect the brake lining, ensuring there is even wear on both shoes, across the axel, all breaks on a tandem. Uneven wear indicates an issue.
   3. Inspect the rollers, pins and bushings for uneven wear. (Note: These components will be discarded along with the springs, but identifying uneven wear will pinpoint problem areas in the foundation brake).
   4. Inspect the spider, brackets and air chamber for damage and security.
   5. Inspect the cam bushings, cam shaft and slack adjuster for wear. (Note: It is recommended that the cam bushings be replaced at each reline, especially if there is indication of uneven wear).
   6. Replace the lining with material that is suited to the application by GAWR, A/L factor and tire size.
   7. Inspect the shoe for stretch, warping, anchor pin fit and roller fit.
   8. Install all new hardware, bushings and other components to ensure they will operate satisfactorily through the next reline.
   9. Adjust the brakes in accordance with the manufacturer’s recommendation.
   10. Road test the vehicle to ensure proper brake operation. (Note: It is recommended that brakes be applied at high input pressure without locking the wheels. A minimum of 20 stops should be made)
   11. Let the brakes cool and check adjustments.

Brake Burnishing

This is critical to ensure proper brake function after brake reline or other major brake maintenance is performed.

APTA Recommended Procedure:

• • 10 snubs
  • From 20 to 5 MPH
  • At approximately 32% decel or every 500’ without stopping
  • After the 10th snub‚ make 1 complete stop from 20 MPH

Better Methods:

• • 20/20/20 – 20 stops from 20 MPH at 20 PSI‚ or 40 MPH to 20 MPH ten times
  • The purpose is to gradually raise the brake temperatures‚ to properly seat all brake components‚ mate the lining to the brake drum to improve contact and to cure the material
  • Measure brake drum temperature to ensure proper brake function
  • Side to side on same axle should generally be within 25° F
  • Steer to rear axle should be within approximately 50° F. This may vary more due to brake valve specifications‚ suspension component compatibility and retarder function