250-500cc 2010 WR300 Rear Brake Keeps Needing Bled

I purchased my 2010 WR300 a while back and haven't ridden it much because the rear brake needs to be bled every time I get on it. When I purchased it, the rear brake didn't work and I thought all I needed to do was bleed the system. So, I replaced the fluid with Motul 600 pulling the fluid and bleeding the system from the caliper with my MityVac pump. I took it for a short ride and the brake seemed to worked fine. The next time I checked it in the garage, the pedal had no pressure again. I bled the system again and it worked again...till the next time I checked it.

I'm figuring I need to rebuild the master cylinder, but wanted to check on this forum for wisdom others might have before I do.

Since I have no fluid coming out anywhere, how do I determine if it's the master cylinder that needs rebuilding, the caliper, or something else? If I am to rebuild it, who best sells the rebuild kits? Or, is there a better solution altogether that I should know of?

Any suggestions would be appreciated.
Thanks.

If no signs of a weep I would blead the system again then apply a constant preasure on the pedal and see if it creaps lower if it does it the master cylinder seals.

Good idea.
I called Hall's today. They don't have a master cylinder rebuild kit, but did have a new "pump" for $96; so I ordered it.

Sure sounds like a master cylinder to me. Halls has everything, I love that place!

hmmm, hope that fixes it. normally the vac makes it a piece of cake. i use plain dot 4 from the auto store tho..swap it out every few years.

Hey just piggybacking this thread so didn't have to start another. Been away on holidays for a coupla weeks, went into shed tonight & no brakes on 300! Pumped rear a few times & came good but front took a fair bit more(had to flick the lever & tap line with lever in) to get working. Second time it's happened past month or so after changing fluid.

Brakes work great when riding but it's obviously getting air in somehow? Fluid was a few years old(motul 660) could it be too much moisture in fluid? Seems odd that both wouldn't work at same time if it was seals but I guess possible.

I've had trouble with the old fluid boiling on the rear brake of my 09 wr250. I'm not a brake dragger and I was dumping the reservoir every 6 months and giving it fresh fluid but it would still boil until I pushed all the fluid out of the caliper thought the bleeder screw a couple of times. I would refill the reservoir each time and pump up the brake so it never ingested air and it's been good ever since. I never push the old fluid back up the line when I replace brake pads but there must have been some fluid in the caliper that had absorbed moisture and lowered the boiling point.

I flushed(reverse with syringe) the old fluid & it was pretty dirty! Rode the last 3 days straight & brakes have been great. Just used some cheap dot 4 stuff but seems to have done the trick. I only recently changed both fluids so it must have been contaminated from sitting for so long? Is it moisture that discolours it?

Yes the moisture makes DOT4 darker in color and is an indicator that the boiling point will be lower than with fresh clear fluid. There isn't a need for special fluid as long as you flush it often, I know of some serious off road pro racers that flush brake fluid after every race.

Cheers boys guess I'll have to do it more often not bi-yearly!