GMC m135 rebuild

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hendersond

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I'll share my experience with a M211 with a stuck brake. It is a similar truck, but with dual wheels. Keep in mind there are 2 wheel cylinders at each wheel. Look on the back side and you will see where the lines attach with a hollow bolt. That will help you envision the orientation of the wheel cylinders. Things are great right up to the point moisture enters the system or the rubber dust boots deteriorate. The pistons inside the cylinders are unique. My truck had an aluminum piston with a lip seal mounted in a groove. It appeared that moisture entered the system from 2 sources. The first I assume was that water contaminated the fluid. It was either rain or humidity, perhaps from the vent??? I cannot say for sure. The fluid leaked out the lower wheel cylinder and out the drum and down the tire. The boots on the lower cylinder was badly deteriorated. I assume moisture created a problem here too. The piston was white with corrosion and appeared swollen. It looked similar to an engine that sat with water atop a piston in the cylinder. It seemed that as the piston swelled it moved outward and pushed the brake tighter. It was a real stinker to get the drum off. I had pulled the axle shaft out and removed the outer wheel bearing. My hope was to make some room for the drum to wobble when beating on it with a 6 pound maul. it takes some weight to transfer energy through that heavy drum to the brake shoes inside.
Next I found the cam adjusters on the back of the backing plate. The adjuster is cam bolts secured with a nut. The "head end is a cam that pushes on the brake shoe and the "thread end" has a jamb nut that can be tightened to keep it from turning when it is adjusted. The thread end has 2 flats on the very end to hold it from turning while you tighten the jamb nut. Those flats also allow you to turn the adjuster from the outside. You can try a wrench, but I needed a vise grip to get ahold of mine. I doubt you will get a full turn from the adjuster. You may only get part of a turn. Turn it both ways, back and forth. Stop in the middle and leave it alone. This is where it is fully backed off. Now get out that big hammer. Envision where the wheel cylinders are located. Estimate where your blow with the big hammer is most likely to push the piston back into each wheel cylinder. Start whacking. You may also need to get a couple pry bars between the drum and backing plate. Good luck!
 
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