2007 yz 450 f rear brake problem

I can't get the brakes to bleed. There is NO leaks and I can pump the brake all day with no pressure build up. I took apart the master cylinder and checked the rubber o rings on the inside of the cylinder. The spring and everything looked fine and like I said there's no leaks at all. The piston in the caliper will go in and out, it's not frozen or anything it just won't build pressure. Should I just by the rebuild kit? Are the o rings just not sealing anymore?

you could try one of those vacume brake bleeder kits they connect to the nipple and suck the fluid through but make sure you keep topping up the master or you will need to start again. I had a similar problem with a car a few years ago and was told the cylinder was cavitating but after vac bleeding it all worked fine

cheers paul

I had the same issue with my 06YZ250.  The rear brake had been gradually getting worse and worse and finally stopped working.  There weren't any leaks that I could find and there was plenty of fluid but they just would not pump up when I bled them.  So I rebuilt the rear master cylinder.  Still won't bleed.  I pump and pumped and never got the pedal to firm up.  I've been bleeding brakes on cars and motorcycles all my 62 years and never had this problem.  I tried raising the caliper above the master cylinder to get air bubbles out.  Tried sucking fluid into the brake line to fill it then filled the master cylinder and caliper with fluid before bolting the line on. Still would not firm up and would not even get bubbles when pumping the pedal and cracking the nipple.  Then I read that some brake fluids don't compress at the same rate.  I had been using DOT 3 & 4 fluid from Orileys.  I got some DOT 4 fluid and also put a caliper rebuild kit in.  Suddenly the brake bled in about two minutes!  Not sure if the caliper seals were the problem or the fluid.  But I think it may be the fluid.  I'll avoid the DOT 3 & 4 from now on.

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