Chronic rear brake problem

I have been fighting a problem off and on with my 98 WR rear brake since July. It acts like the system will build up pressure causing the brake to begin to drag excessively and will rapidly progress to the point of locking up the rear wheel. I have always been able to correct the problem on the trail by cracking the bleed valve and letting a small amount of fluid out to relieve the pressure. Once this is done the problem will not reoccur during the rest of the ride. This has happened on back-to-back rides, then sometimes I can go on five or six rides over the course of a month or more with no problem. It usually occurs within the first few miles of the ride and I have had this happen twice where I have intentionally not even touched the rear brake at all during the ride. I don’t over fill the reservoir. The rubber reservoir cap gasket and master cylinder are in good shape and I have rebuilt the master cylinder once (new seals, plunger, etc.). The pedal linkage is adjusted properly and lock nut is tight. I have changed out the pins that hold the pads and always make sure they’re clean and smooth. I have replaced the pads frequently. The last pad change I also installed new facotory metal shims that mount between the pads and caliper. Up to this point, these weren’t installed from the bike's previous owner. I verified the brake rotor is not warped. I have replaced the caliper piston and seals twice now. The caliper bore has no scoring/wear marks and the pistons I have replaced seem to be in good conditions. I have also switched from a decent quality DOT 4 brake fluid to Motul 600 and even installed a Fastline braided hose. Last weekend was the last straw. I ended up dropping 5 points at the first check at the D37 Ghost Riders Enduro because of this problem. I am getting pretty sick of it and I’m on the verge or replacing the master cylinder and/or caliper. Before I go to this extreme, does anyone have any ideas on a cause/solution to this problem? The only common symptom is that if this is going to occur, it will tend to happen within the first ten miles of a ride. This makes me believe that as the master cylinder is warming up (due close proximity to header) and something inside changes causing the plunger to move up into the cylinder like the brake is being applied. Has anyone else seen this? Is there something I’m overlooking?


brake hydraulics is where i did half of my appreniceship, remove your master cylinder, clamp it in your vice in operating position, pull a peice of wire out of your wire brush, fill your master cylinder and pretend you are bleeding it, you should have a fill port and a releif port from the reservoir, both these ports should be clear,( check with your wire) when your push rod is released, if this dosent happen your seals are wrong, i suspect your master cylider is at fault so check this first and let me know how you go.

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