Rear Brake Locking Up on YZ 426

I bought a used 2000 YZ 426 and when i ride it for more than10 minutes the rear brake locks up,I took the master cylinder and caliper apart cleaned them looked for problems and didn't see anything wrong,I made sure the master cylinder worked all right and cleaned up the caliper and reassembled them.Same thing today I ride about 10 minutes and the rear wheel locks up,if I crack the bleeder it loosens up again. Any and all suggestions are appreciated as I am a little out of practice with bikes nowadays i have spent the last 12 years building and racing late model stock cars so I am somewhat mechanical.

Thanks Jim Cooksey

It sounds like the caliper seals may be the culprit,

Did you actually take the caliper and master completely apart and inspect the rubber/pistons inside? look for debris?

or did you just take them off the bike and spray brake cleaner around?.

Yeah I took both the master cylinder and the caliper completely apart and cleaned double checked everything,the caliper was so hot I couldn't touch it after 10 minutes of riding today ,but the second i cracked the bleeder it released and was good for another 10 minutes.

Check the return orifice in the master cylinder!

I had the same problem with my brothers 82' Honda street bike. The return orifice was clogged so when you released the brake lever the pressure would not relieve into the reservoir so the brake caliper would not release (it was his front brake and my knee joint was "loose" for about six months after doing an unexpected stoppie and twisting it while trying to find out what was wrong).

Sounds like the pedal free play isn't enough. You must be certain the master cylinder piston is returning all the way to the end, if it doesn't, the "replenish" port will remain closed and it cant "dump" pressure in the line. Then as the pads heat up from the extra pressure, the fluid behind the caliper piston warms up, thus further applying the brakes, thus generating more heat, thus applying harder... so you get the idea, its a snowball affect. Adjust the linkage between the pedal and master cylinder, just make sure the pump stroke is all the way out at rest. Does the brake pedal still have its return spring in place? This will also do it.

I got the same problem with my rear brake and I had lost the spring attached to the pedal.It works perfect in the garage then the bike is clean but in the dirt... Check the spring and the free play of pedal.

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