Rear Brake Issue - Need Input

[blakeNY9 0]

Had a rear brake failure on the trail today and I'm at a loss as to what the problem is.

What happened... rear brake working fine at the start of the day. During an uphill, laid the bike down, re-mounted and continued. At the top of the hill going down a decent, noticed I have no rear brake action, rear brake lever going completely thru the stroke without engaging rear brake. Stop and investigate, no fluid leaking, no visible damage, reservior and caliper cool to the touch. Get the bike home, check fluid level, it's full. Open bleeder valve, pump lever, weakly spits some fluid and some air. Keep pumping and now it's just weakly spitting air then sucking air back in. Reservior remains full.

I think that covers it. Seems to me some internal is not engaging or working properly? Thoughts? Anyone experience anything like this before?

This is on a 08 KTM 300 XCW.

Thanks for your assistance.

[robertaccio 412]

reservoir air gap, laid over and air got pumped in into pressure side of system in past the master cylinder (most likely cause). I would back bleed it completely with a pressure fluid bleeder and start from there, that will most likely fix the issue. But the gripe and what is doing still could be a bad master cylinder seal as well. step by step and step 1 is a full system bleed. if that doesnt fix the issue tear into the master cylinder, there are brembo rebuild kits. Oh yea make sure all the mechanicil linkage are intact, Ive seen the brake pedal push rod pop out of the master cylinder when things are out of adjustment

[hobiee 28]

yup. What Robert said. Check adjustment on plunger on backside of pedal. Back bleed it to force the air bubble out. Master cylinder rebuild is a snap if it needs it.

[blakeNY9 0]

Thank Robert and Hobiee. Tried a vacuum bleeder and that pulled a little fuild then nothing even though reservior is full. Tried pumping lever while pulling with the vacuum bleeder and still nothing.

Syringe will be here tomorrow and I'll try the reverse bleed technique next but something just doesn't seem right. Seems like I should order the rebuild kit now since it's a week lead time!!!

[hobiee 28]

Do you have a crushed/pinche'd line somewhere?

[350thumper 336]

Sometimes debri gets lodged and stuck into the tiny hydrolic orifices ..when you get the master cyl off (and apart) use compressd air in reverse with brake cleaner to blow the pasage ways clean..its easy to tell if the master is working by removing the line and putting your thumb over the output hole while you pump it.

[blakeNY9 0]

No crushed or pinched line. If there was a blockage (or pinched line) wouldn't there be resistance in the brake lever? My lever goes completely thru the stroke with no brake action.

[blakeNY9 0]

No crushed or pinched line. If there was a blockage (or pinched line) wouldn't there be resistance in the brake lever? My lever goes completely thru the stroke with no brake action.

[350thumper 336]

Not if it was blocked at the intake to the master,

A tiny bubble can cause the same effect .

Stupid micro bubbles... they take forever to move

[350thumper 336]

I call this tool the supersucker5000

Many many hydrolic systems were serviced with this

And it stills goin. Stupid smartphone.

[blakeNY9 0]

Got the syringe today, tried the reverse bleed and it wouldn't take any fluid. I was putting a lot of pressure but it won't fill. Yes the bleeder valve was open, I triple checked and yes the reservoir cap was off. Took master cylinder and line off the bike. Line is clear, bleeder valve is clear. So it's got be something with the master cylinder...??? Hopefully I can find parts for master cylinder rebuild before the weekend.

Any other thoughts?

[hobiee 28]

If you can't find a rebuild kit in time I could take the master off one of my bikes to let you borrow . I can only ride one at a time.

[blakeNY9 0]

Thanks Hobiee I appreciate that.

So after the first failed attempt at reverse bleed, and taking off the master cylinder and hose and got all the fluid out, figure why not try the reverse bleed again with a completely empty system. It took fluid this time, filled up the reservoir. Tighten everything up, pushed the lever and same thing, it goes all the way thru the stroke without engaging the brake. Pushed lever, cracked bleeder valve, some fluid spit out, tighten bleeder valve, pushed lever again, cracked bleeder valve, again fluid is coming out with some pressure. Not sure if its the proper amount of fluid or pressure coming down the line but it's giving some pressure at least.

Could the problem be in the caliper? Is it common that seals in the caliper have gone? There is no leakage (not sure if there would be any). Is it possible that the master cylinder is pushing fluid but not enough i.e. degraded but not completely failed seal?

Appreciate anyones insight if they have experienced this before. Trying not to just start buying part after part chasing this down.

[350thumper 336]

Bench test the master cylinder by putting your thumb over the output hole , if its pumping properly it will squirt under your thumb. Squirt...

Or like hobie said mount a different master cylinder on there that will test the rest of the system.

I have the same problem with my rear brake on my husky but I really dont care since its 70/30 rear.

But then I think brakes are over rated ... because all they really do is slow you down....