Rear MC - no pressure

[txtriathlete]

I've spent the past few days getting acquainted the rear brakes on my 80 Special. As of right now I have bled through about a full bottle (big bottle) of brake fluid with a mighty vac. Still can't get it to hold any pressure - basically a dead pedal. I don't see any leaks anywhere which makes me think its something inside the MC. No air in either end of system, at least that I can get to bleed out.

My Haynes manual doesn't show the MC piston and seals in detail, its possible I have them reversed or in wrong somehow. The rebuild kit had 2 rubber seals, metal piston rod, a small metal cap, spring, solid disk and c clip all of which I replaced in the order they came out.

Does anyone have closeup pictures of the rear MC piston with seals? Its a pretty simple system but I can't get it to "push" fluid - seems the fluid is just bypassing and not compressing. Maybe a seal is out of place or in backwards? I've had the MC apart twice now.

What am I missing?

[Ken Talbot]

Have you seen this tip yet from the tech articles section? :-)

[txtriathlete]

Have you seen this tip yet from the tech articles section? :-)

Yep. That's for the front MC piston. Rear is completely different. Thanks, Greg

[Radioguylogs]

The rear is super easy to bleed, so I doubt that is your problem.

Perhaps the cup in your master cylinder isn't springing out against the walls of the cylinder.

Could you try a bit of back pressure,... maybe put a hose on the caliper bleed screw, open the bleed screw, and blow backward a bit while pumping?

[DiverRay]

You DO need to "bench bleed" the master! If you don't, you will never get it to work correctly. With the brake hose off the master, put a finger over it while you depress the piston. Let the air escape, but seal with your finger BEFORE letting off the piston. Repeat until pressure and fluid come out, then hook everything back up and bleed the system.

[txtriathlete]

You DO need to "bench bleed" the master! If you don't, you will never get it to work correctly. With the brake hose off the master, put a finger over it while you depress the piston. Let the air escape, but seal with your finger BEFORE letting off the piston. Repeat until pressure and fluid come out, then hook everything back up and bleed the system.

I saw this on another post earlier today doing a google search. will give that a try.

Thanks, Greg