250-500cc WR300 rear brake trouble

Hello all,

Im having a ton of trouble with my rear brake on my 2014 WR300 (yes it is a 2014). New pads, fluid, pads are free on the sliders etc.

Every time I ride, the rear brake ends up completely not working after 30 or so minutes of riding. Ill take it home, bleed it, make sure the rear master is working correctly, and theyll start working again. next time I ride... same thing. Ive rebuilt the master cylinder and make sure NOT to overfill the reservoir and all that. I also notice that if it gets the least bit of water on it, it quickly fails after that.


Anyone have any advice on what it could be? Im tempted to replace it all (master, caliper, pads and line)

Thanks, Ill try adjusting the pedal and see if that helps. Not sure if it will as the brake work initially and then completely lose pressure after a short ride.

Must be taking in air somewhere. Replace all seals in master cylinder and caliper. Not sure about the water though.

Thanks guys. Much appreciated. Im going to replace the caliper, master, and line. THatll fix it.

I had the same issue with mine. I tried rebuilding the master and it still did it. I gambled and bought a used master and it fixed the issue.

just for grins, try taking the master cylinder off the frame (brake hose still connected) and raising it as high as possible. IOW, raise it above the height of the swingarm pivot loop/bend in the brake line. secure it (piece of wire?) with the hose down. Take a screwdriver or whatever and force your brakes apart- basically move the slave piston in all the way. if there is a hidden air bubble at this troublesome spot, this should get it; also since the actuating rod is out of the m/c piston, the piston should be retracted all the way, uncovering the relief port if it wasn't before.

then, if this doesn't work- buy the new stuff... 'cause you're talking a lot of $$.

also, I am not sure you mentioned this (sliders?) but make sure the caliper is free to move on it's pins (and I am NOT taking about the brake pads or their pins here, but both halves of the caliper- mount & cyl. body). Use silicone grease or brake grease in the rubber cups & pins. this is overlooked many-many times, but can cause no end of weirdness. both the caliper and the slave piston must be able to move independently and the same distance.

nelly, if you want to get rid of that stuff- I'll pay for shipping. Yeah, I think it's still usable. I will make an inspection & diagnosis, and if I find anything- I will let you (and everybody else, too) know what the problem was.

Pop the piston out carefully with compressed air(CAREFULLY! Air gun nozzle on bleed nipple) to ensure no air trapped behind. I had issues 6 months back similar deal, popped piston out after new MC rebuild kit failed to fix problem & voila all fixed!!!

Bloody exy to replace everything & overkill!

shawbagga, I am not sure I'm getting this. You're recommending pulling the slave (?) piston out to check for air behind??

if air is in the back of the piston, lay the bike down on the sprocket side (front wheel slightly uphill if possible) should cause this bubble to migrate to the bleeder hopefully ...or up to the swing air loop if there's a lot (arggghhhh). give it 5 minutes on the side.

some people will fill the slave cylinder up on the bench to mitigate this; I don't usually do this.

I ended up grinding a notch in my brake pedal to clear the weld on the frame, it helped by giving me more brake travel and adjustment and make room for my size 14 boots.

Also if you're popping the piston out with air be sure to put a piece of plastic or wood where the brake pads go to act as a backstop for the piston and keep your fingers out of the way... I've seen pistons stick and then let go like a gunshot, when this happens it can damage the piston and your fingers if they happen to be in the way. The same safety precaution applies to automotive brake calipers.

Yep what Doug said. Couldn't get brakes to bleed no matter i tried(normal/reverse/raising MC/new MC kit). Popped piston out(with hard plastic in caliper receptacle) & came good. Air just trapped i guess!

Caliper piston

thanks guys. I ended up buying another caliper and MC from another member. Lets hope replacing it all works. lol