At each wheel, the brake actuating push rod of the air brake chamber connects to an automatic slack adjuster mecha- nism, which acts as a lever to turn the brake assembly’s S-cam shaft.
As the friction surfaces of the brake shoes wear, they grow thinner, and the clearance between the brake shoes and drum increases. If this situation were left uncorrected, the brake chamber push rod would have to travel an ever- increasing distance in order to actuate the brakes and fre- quent manual adjustment would be necessary to remove this excess travel. The role of the automatic slack adjuster is to compensate for the brake shoe wear by acting as a ratch- eting mechanism, much like a ratchet wrench, keeping the linkage travel within normal tolerance.
Meritor slack adjusters are standard on the Blue Bird Vision. Haldex adjusters are on some units as an option. The two types perform the same function, but by some- what different internal mechanisms.
In Meritor slack adjustors, the ratcheting function is performed by a pawl which engages the notches of a toothed adjusting sleeve which rotates as brake lining wear occurs. The spring-loaded pawl can be manually released by pulling a button on the outside of the slack adjuster body.
On Haldex adjusters, the internal ratcheting action is performed by a one-way clutch on the shaft of a worm drive gear which rotates as brake lining wear occurs.
Appendix 12 contains more detailed information on the Meritor automatic slack adjusters. Appendix 13 contains more detailed information on the Haldex automatic slack adjusters.
Adjustment
The slack adjusters on the Blue Bird Vision are designed to be self-adjusting. The only times at which manual adjustment should be necessary is when initially setting the adjusters after reassembling the brakes following service procedures such as shoe replacement. If brake travel is found to be out of range, always be sure to find the root cause. Making manual adjustments of the slack adjusters is probably only affect- ing a symptom, and not correcting the actual cause of a problem.
Thorough instructions for making the initial slack adjuster settings after servic- ing the brakes are contained in the two appendixes mentioned above.
Removal of Meritor Slack Adjusters
Meritor slack adjusters are mounted on the splined shaft of the S-cam, and secured by an outer diameter circlip. The end of the actuating arm is connected to the brake chamber push rod clevis by two clevis pins and clevis pin retainer clips. To remove:
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1. Park the bus on a level surface. Stop the engine. Chock all wheels securely to prevent movement in either direction. Means other than brakes must be used to prevent vehicle movement.
2. If the bus is equipped with air suspension, support the frame rail securely with safety stands.
3. Open all three tank bleed valves to drain the air brake system to 0 psi. 4. Manually disengage the spring brake as described above in Manual Spring
Brake Disengagement.
5. Remove the two cotter pins and clevis pins that connect the pushrod to the slack adjuster.
[WARNING] Do not re-use clevis pin retaining clips after removing them. Always replace used clevis pin retainer clips with new ones.
6. Pry the spring-loaded pawl button outward to release the ratchet mecha- nism of the slack adjuster. While holding the pawl outward, use a wrench to turn the adjusting hex head on the bottom of the adjuster clockwise. This will cause the slack adjuster to rotate. Turn the nut until the slack adjuster arm is clear of the pushrod clevis.
7. Use outer circlip pliers to remove the circlip securing the adjuster assembly to the S-cam shaft. Note the number and assembly order of spacing washers on either side of the adjuster as you remove the adjuster from the shaft. Installation
Reinstall the Meritor slack adjuster by reversing the removal procedure. After install- ing, make the initial setting of the slack adjuster by following the instructions in Appendix 13.
Removal of Haldex Slack Adjusters
Haldex slack adjusters are mounted on the splined shaft of the S-cam, and secured by an outer diameter circlip. The end of the actuating arm is connected to the brake chamber push rod clevis by one clevis pin and a clevis pin retainer clip. The adjuster’s control arm is fastened to a slotted hole in the anchor bracket. To remove:
1. Park the bus on a level surface. Stop the engine. Chock all wheels securely to prevent movement in either direction. Means other than brakes must be used to prevent vehicle movement.
2. If the bus is equipped with air suspension, support the frame rail securely with safety stands.
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3. Open all three tank bleed valves to drain the air brake system to 0 psi. 4. Manually disengage the spring brake as described above in Manual Spring
Brake Disengagement.
5. Remove the clevis pin retainer clip and clevis pin which connect the pushrod to the slack adjuster.
[WARNING] Do not re-use clevis pin retaining clips after removing them. Always replace used clevis pin retainer clips with new ones.
6. Use a wrench to turn the adjusting hex head on the bottom of the adjuster clockwise. This will cause the slack adjuster to rotate. Turn the nut until the slack adjuster arm is clear of the pushrod clevis.
7. Use outer circlip pliers to remove the circlip securing the adjuster assembly to the S-cam shaft. Note the number and assembly order of spacing washers on either side of the adjuster as you remove the adjuster from the shaft. Installation
Reinstall the Haldex slack adjuster by reversing the removal procedure. After install- ing, make the initial setting of the slack adjuster by following the instructions in Appendix 13.