Is my rear master cylinder shot?

[Skiforce75]

I changed the lines on my brakes and then bled them using a vacuum pump. The rear will not build pressure and when the vacuum is applied at the bleeder, the reservoir drains without the brake being applied.

Because of this, it seems to me that a seal in the master cylinder would be blown allowing fluid past it regardless of the position of the brake.

What say you?

Thanks in advance.

[bogieboy]

the system is designed to allow brake fluid to go into the line without anything being applied. the trick with vacuum bleeders is that you can suck the master dry real quick... on a rear brake i would hand pump it up till you have some sort of pressure, then crack the bleeder, let air out, repeat till you get clear fluid... vacuum bleeders are more for car applications where the lines are 10+ ft long....

[Skiforce75]

Okay. However, with nothing but fluid in the system there is absolutely no pressure and it will not build with even 100 pumps.

[bogieboy]

Originally Posted by Skiforce75

Okay. However, with nothing but fluid in the system there is absolutely no pressure and it will not build with even 100 pumps.

then yeah... master is shot....LOL unless theres an issue with the linkage...

[Skiforce75]

Originally Posted by bogieboy

then yeah... master is shot....LOL unless theres an issue with the linkage...

I am inferring that there would be leakage if there was a linkage problem.

[pete]

rear brakes are usually not to much of a issue...
BUT
if the master cylinder is higher than the caliper it
can give problems bleeding the system..
remove the caliper… raise it as high as you can
put something between the pads and try bleeding it..
you want the bleed nipple the highest part of the system..
air goes up...


..

[bogieboy]

Originally Posted by Skiforce75

I am inferring that there would be leakage if there was a linkage problem.

not necessarily. the master would still hold fluid if the linkage was out of adjustment. only thing that would make it leak would be the outer piston seal being shot. if the inner is shot from using the wrong type of fluid or just old seals being worn out, the master would hold fluid, not leak, but still not work. (learned that the hard way...power steering fluid does NOT work in a brake system...LOL blew a brand new front master cylinder in 20 min...why do the bottles have to look the same, and companies use labels that can fall off... )

[Kdburtch]

tried vacuum bleed on mine and the rear brake did not work at all. Then bleed it with the pedal and now it seems operational. I would suggest trying the old way as a troubleshoot.

[Megadan]

Originally Posted by Skiforce75

The rear will not build pressure and when the vacuum is applied at the bleeder, the reservoir drains without the brake being applied.

This is perfectly normal. I have a pneumatic vacuum bleeder that I use to drain brakes before removing lines or other major work. The reservoir feeds into a chamber that the piston then compresses the fluid that then travels through the line. When the piston is at rest (not compressed) the fluid will easily pass straight through and out the bleeder.

As far as putting new fluid back in, I often find that a combination of manual bleeding and vacuum bleeding works the best to get air out of the system. I will usually start by manually pumping fluid through the system to prime it, then following with a vacuum bleed to draw any trapped air works best, but you have to be on top of the reservoir level. If you can tie the caliper up above the master when doing this it is far more effective.

[NzBrakelathes]

Don’t these bikes have the hose from master to wee pot behind the right side panel?

Then fluid is way up n bleeding will be easy enough
Open bleed screw with wee hose attached n watch the air come out slowly
Also be sure you have free play in the pedal

[Skiforce75]

Originally Posted by pete

rear brakes are usually not to much of a issue...
BUT
if the master cylinder is higher than the caliper it
can give problems bleeding the system..
remove the caliper… raise it as high as you can
put something between the pads and try bleeding it..
you want the bleed nipple the highest part of the system..
air goes up...


..

I did all of this. No luck. Thanks though!

[NzBrakelathes]

Originally Posted by pete

rear brakes are usually not to much of a issue...
BUT
if the master cylinder is higher than the caliper it
can give problems bleeding the system..
remove the caliper… raise it as high as you can
put something between the pads and try bleeding it..
you want the bleed nipple the highest part of the system..
air goes up...


..

Incorrect
It self bleeds if fluid higher then outlet