Losing rear brake...boiling?

For some reason my rear brake keeps losing pressure at what seem to be random times. I rebuilt the master cylinder last spring and have bled them several times. What is throwing me is that they will hold pressure for hours/days and then suddenly be gone (of course just about the time I really need them).

 

Here is what I have done so far: rebuilt the master cylinder with aftermarket parts. Bled them several times. Rebuilt the master cylinder again, this time with Yamaha parts, bled them. Replaced the rotor (thought may it was warping when it got hot). 

 

At this point I am thinking that maybe the fluid is boiling. I used fluid (at least on the last rebuild) that I bought off the shelf at the parts store (not a name brand).

 

I don't see any leaks, but I did notice that the top of the master cylinder where the cap bolts on was a little damp....

 

 

Any ideas...?

 

 

clean up all of your brake parts go for a ride and hammer the brake, this will help highlight if any fluid is leaking

 

this happened to me on my snowmobile, couldn't figure out a soft brake condition and turned out to be a small hairline crack below the MC i couldn't even see it

The only reason you should have any boiling problem is if there is water in the brake fluid.  It's important to use DOT4 fluid in the system, too.  If you didn't, drain the system, refill with DOT4, then drain and refill again. 

 

My guess is that you simply need a new master cylinder.

The only reason you should have any boiling problem is if there is water in the brake fluid.  It's important to use DOT4 fluid in the system, too.  If you didn't, drain the system, refill with DOT4, then drain and refill again. 

 

My guess is that you simply need a new master cylinder.

Would a worn cylinder bore account the inconsistent pressure loss?

Possibly.

You say the problem occurs at random times. When the brake goes soft are you able to pump it back up, or does it need to be bled? And after that happens, do you get air in the first couple of pumps or does it take a while to reach the bleed?

I think it is heat related. It's not done it cold. It's only after going for a bit. I want ahead. I am leaning toward Gray's idea. It makes sense if the bore is borderline it would allow fluid to pass the seals when it gets hot. I would think that is it was just air in the line I would have to pump the brake to get out working after sitting a while.

Sorry for the spelling stupid auto correct on my phone.

If you don't have a leak and your not seeing air during bleeding it can really only be a couple of things. Most likely the matter cylinder is in fact worn, or your wheel bearings are loose causing the disc to spread the pads slightly under any side load.

My friends 2000 426 did the same thing. After looking at my 99 400, he found he was missing the pedal return spring. We found a heavy spring to use as a return spring, and to my knowledge his issue has not returned.

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