When I started with the school district the normal procedure for brake repair was to machine the drums and linings even if they were new. We have a brake shoe lathe that slides onto the axle and turns the shoes to achieve 100% contact between the shoes and drums. The drum lathe does the same job and machines the drum until there is an even finish.
The only objection against machining the new shoes and linings every time was the man hours to get it done. We leaned towards just replacing the drums and shoes without machining on our buses and had no problems doing that. Most of our buses have 7 to 8 5/8 inch shoes and it was a quick re and re and they were out the door.
Machining Brake Shoes in the video below shows how it’s done with the tools we have in our shop. This is on a pusher school bus with 7 inch brake shoes because of the higher GVW than the smaller wheel chair bus we’re working on right now.
After replacing the front drums and 5 inch QPlus shoes on this wheel chair bus there was a wobble in the steering wheel with the brakes lightly applied. Usually new drums and shoes are a sure thing with out any trouble on the road test….not in this case. The smaller shoe size I believe is a factor and this is not the first time this kind of problem has occurred.
Past Experience Same Model Bus
A short time ago we had a wheel chair bus like this one with the exact same specifications that had a bad brake squeal. After changing shoes and linings it was worse with a vibration and brake pull. After machining both drums and shoes the problem persisted. We decided to replace the auto slack adjusters and that made a vast improvement. This was the first brake job for this bus so maybe the aftermarket brakes and shoes may not have been up to standard?
With this present brake job we replaced the auto slack adjusters as part of the initial repair. In fact we are now including slack adjuster replacement with all of our brake jobs now so there is no question with push rod travel and imbalance problems. The smaller brake size might be part of the problem since it does not have a lot of contact area and seems to be much more sensitive with brake pedal feel and driveability.
The Road Test Will Be The Ultimate Factor
Once the brakes and shoes are machined we will take it out for a drive and see what happens. There will definitely be an improvement since we have already made a few passes on one of the new brake drums on the lathe. It’s out from being perfectly true by at least .020 inch. I’m hoping this will cure the brake shimmy and there will be a happy ending after all this extra labour.