Brake fluid won't enter master cylinder

[keelerb]

Trying to trace a problem in rear brake - wouldn't bleed. Caliper seems fine, as does line from master cylinder to caliper. Master cylinder itself seems to spit out any fluid in it when brake pedal is pushed. But I can't seem to get fluid to run from the reservoir into the master cylinder. The reservoir drain hole is clear, as is the line to the MC. But fluid won't run into the MC.

I tried blowing brake fluid into the MC, then air - neither will enter. Now maybe there's some sort of check valve there (but I would have thought it would let air/fluid ENTER from the reservoir)?

Can anybody offer any clues?

Thanks - Brian

[PGlaves]

Trying to trace a problem in rear brake - wouldn't bleed. Caliper seems fine, as does line from master cylinder to caliper. Master cylinder itself seems to spit out any fluid in it when brake pedal is pushed. But I can't seem to get fluid to run from the reservoir into the master cylinder. The reservoir drain hole is clear, as is the line to the MC. But fluid won't run into the MC.

I tried blowing brake fluid into the MC, then air - neither will enter. Now maybe there's some sort of check valve there (but I would have thought it would let air/fluid ENTER from the reservoir)?

Can anybody offer any clues?

Thanks - Brian

Make sure the piston fully retracts. It can block the holes.

[AnnapolisAirhead]

Which bike? Handle bar MC or under tank?
I'm not sure what you mean when you say the caliper and the line seem fine? How did you test them?

There is no check valve. Hard to tell, but it could be that something has caused the spring in the MC to get stuck and not return to its resting position. You could also have something clogging the reservoir. Does it leak out at all?

Oh, and to ask the obvious, was the bleeder valve open when you tried to squeeze the lever? When you squeeze the lever (with the bleeder cracked open and with brake fluid in the MC reservoir (don't let it go empty and cover your painted surfaces), what happens? Is it stiff or does the brake lever just collapse like there is no resistance?

I've finally rebuilt my calipers, new stainless brake lines, rebuilt the MC. It's a pain, but ya gotta get em right.

[keelerb]

Reservoir and line to MC are clear. I'll have to check the piston.

Thanks.

[keelerb]

Hmm, fluid will spit out the exit hole of the MC when the pedal is pumped, so I would think the piston must be moving...?

[keelerb]

Problem is, with a vacuum bleeder on the bleed screw, I can't suck fluid through. Removed the caliper, I can such fluid through that. Same, through the connection to the MC. And the reservoir and line in to the MC are clear. So the problem seems to be in the MC, and that, the fact that the spigot to which the reservoir line attaches will not permit air or fluid to pass into the MC.

[lostboy]

It sounds like the vent port in the master cylinder is plugged. Assuming the lever has not been adjusted and that the linkage is free, this is often the source of brake problems. This port is about .015" in diameter and corrodes shut easily. Remove the line that screws into the master cylinder and attach your vacuum bleeder to it. You'll probably find that you can't pull anything out of the master cylinder.

The tough part is clearing the hole. Remove the M/C and disassemble it, then look down the bore. You'll see two holes, one much larger than the other. Using a piece of wire bent at 90 degrees and sharpened, poke at the hole until the wire clears the hole.

Or you could just replace it.

[keelerb]

Yup, got the MC off, removed the piston, and now that vent hole will flow, so I think Paul was right that the piston was sticking.

Only thing is, the sucker popped out of there quickly enough that I couldn't see where two washers/seals fit - one is a simple metal washer and the other is a seal that is "flat" on one side (with part number on that side) and lipped around the interior diameter on the other. Not sure which goes on the piston first, nor which side of that seal faces out....anybody know?

Thanks - Brian

[keelerb]

Anybody?

[jbcollier]

You don't say what bike you have. Here is a parts break down of the R100RS rear master cylinder:

http://www.webparts.com/parts/33959.jpg

I would put in an overhaul kit.

[keelerb]

Sorry, 84 R100RT. Parts fiche looks same as for my bike. But not all the parts of the piston and associated parts show - like, where's the spring? And those two seals/washers don't seem to be there.

Anyway, cleaned it out, reassembled as best I could, it's working. Just got to bleed the sucker now.

Yeah, I should rebuild the MC, and sooner or later I will. Just not today....

Thanks all

[AnnapolisAirhead]

Not sure if you've resolved all this, but I found a better price on a MC rebuild than when I had to do mine (Murphy's Law).

http://beemerboneyard.com/macyreki1.html

[lostboy]

The spring goes on the front of the piston so it can push the piston back. There's a rubber washer and cup washer at the rear: you'll note that the rubber washer fits into the cup washer. The rubber washer is intended to keep dirt out of the cylinder and the cup washer to retain the piston assembly. The cup washer's flat side faces out. It's supposed to be an interference fit and is really only good for one installation. It is tapped in with an appropriate tool-I use a Snap-On deep 11mm socket.