brake question from a newbie..., my search didn't answer my question.. Options

[red914]

first i will try to describe the problem in a meaningful way. i have newer calipers in the rear (from PO). the right rear has developed a drag problem. it does not seem to release, and hence, drags, heats up, etc. I have tried to decipher the haynes manual, but to no good result. does the brake proportioning valve have anything to do with this? how does one adjust the venting? i messed with that a bit, but the allen screw and lock nut allwo brake fluid to seep out of from arond them when screwed in, and only seem to tighten up when run out. none of which, by the way, effects the drag at all.

i realize this is a rambling question, but i am in over my head at this point. any knowledge out there?

[davep]

The rubber lines can swell closed such that the MC can force fluid through the restriction, but it does not allow fluid to return back. This is a very fast way to wear pads out, so if the pads are worn more on this side it is a good indication of this problem.

The pads can rust into place and not easily move in either direction. If after removing the pins and the pad spreader, the pads are very difficult to remove, then this could be part of the problem.

Using the adjuster screws, either inboard or outboard, should move the pistons provided the pistons are not rusted in the bores. Without the pads in place, the adjusters should move the pistons smoothly and easily. If the calipers have not been properly maintained, then the adjuster mechanism that is locked into the piston could have been pulled out of its locked position, and the adjuster will not work properly. This happens when the pads wear out without the adjusters being used to periodically reset the venting clearance. A good reason to adjust say every 10000 miles.

The piston seal, the square profile O-ring, does more than seal the piston to prevent fluid leaks. Half of its function is to distort outward as the piston is forced out during braking, then to pull the piston back afterwards. If the bore and/or piston is too rusted, the O-ring is unable to pull the piston back and provide the venting clearance. Using the adjusters to set the venting clearance is just accomplishing much the same thing , but with the addition of properly positioning the spring assembly in the piston to assist with the pull back. So both the O-ring, and the spring mechanism retract the piston after braking, and the wobble in the rotor pushes the pads back.

A seized parking brake cable can also keep the brakes on. In the 914, the parking brake basicaly uses the inboard piston to push the inboard pad against the rotor. If the rotor flexes a little, then the rotor will push against the outboard pad. Thus the braking action is achieved. When the calipers lever arm is pulled, an internal cam pushes on the adjuster assembly forcing it outward. Exercising the lever should produce noticeable, bidirectional, movement of the inboard piston.