Brake Problems!

A friend of mine is having problems with his rear brakes.. He has a 99' WR400..He just put a rebuild kit in the rear master cylinder and it has new fluid.. After just a couple of miles into the trails his rear brakes lock up, he can relieve the pressure with the bleed nut and it will work fine for another couple of miles and it will lock up again...It was locking up before the rebuild..

Any help would be great..



make sure the pedal is going all the way up, and that there is no downward pressure on the pedal.

There is no air space in the rear master cylinder fluid resiviour.

You should not fill the resivior all the way full. leaving room for fluid expansion.

as the rear brake heats up the fluid expands and if the resivior is full it just expands the caliper pistons making more heat and expanding the caliper pistons more.

One more thing. Check to make sure the rear rotor is not bent. As you apply the brakes the first few times the pads will adjust in and if the rotor is bent you can get the same symptom.

Thanks everybody we are still working on it...


there are two more issues that can cuase this.....

1. swollen caliper seals make the caliper too tight not allowing the fluid to return to the master cylinder

2. the rear brake line tubing is bad somewhere and will let fluid in but not back out.

is this why you rebuilt it in the first place? Is it fixed yet????????

Check the rear brake line, sometimes they swell and prevent fluid from flowing back to releive pressure on the piston. Saw this in my Porsche once.

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