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1991 XR250L rear brakes locking up

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After I take off and hit the rear brake a few times while riding it starts to lock up. So what I do next is crack open the bleeder valve just a little bit to release the pressure and it unsticks the brakes. I also crack open the banjo bolt hose that leaves the master cylinder and the brakes unlock after I open it to. I rebuilt the Caliper, master cylinder and just replace the hose and its got new brake fluid. My guess is when the brake fluid heats up it makes either the caliper or the master cylinder lock up, because I can push on the brake lever and bunch of times when its cold and it doesn't lock up.

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How full are you filling the master cylinder with fluid? Don't fill above the MAX line on the reservoir. Make sure the rubber gasket in the reservoir cap is not extended.

 

What DOT brake fluid are you using?

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How full are you filling the master cylinder with fluid? Don't fill above the MAX line on the reservoir. Make sure the rubber gasket in the reservoir cap is not extended.

 

What DOT brake fluid are you using?

I've been putting in dot 4 and I also just bought a new diaphragm ("rubber gasket") for the reservoir and its not extended. I just got back from riding it and sometimes it won't lock up for a while, then all of a sudden it does. I'm thinking my master cylinder is bad, because when I crack it from the banjo bolt by the master cylinder the brakes unlock. And when I do that It won't lock up again till like the 6 stop sign. I have the level at under half between the two lines.

Edited by mrbean_100

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Was it doing this before the master cylinder rebuild? Or did you rebuild it because of this issue?

Even tho your fluid level in the res is between min and max when cold, have you taken the cap off the res when this problem happens to see what the level is in it at that time?

Could you have installed one of the cups on the master cylinder pushrod backwards?

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