[Simple Man]

Don't drive my 68 GTO much, because the brakes are CRAPPY! Replaced M/C, with a rebuilt, rebuilt calipers, wheel cylinders, and SS brake lines about 2 years ago. Brakes were acceptable. Drove it last fall, brakes were spongy. About 3 months ago, got in, and the pedal went to the floor. Front reservoir was dry. No visible leaks anywhere.Refilled the other day, tried bleeding calipers, no pressure. Thinking M/C is kaput.

Who makes a decent M/C for these cars. 4 piston calipers up front. Tired of the AutoZone Advance O'Reillys rebuilds that don't last. Does Wilwood, or anyone like that make a bolt up M/C, for these cars?

Also, on the side of the M/C is a valve of sorts, that goes in the line, from the front reservoir, to the prop valve. What purpose does that valve have? Residual pressure for the calipers?

[webfoot]

Possible it is leaking into the brake booster, look for drips down the bottom center of the booster.
RockAuto sells brand new AC delco master cylinders. In fact I have an extra I could sell you quite reasonably, bought one and never ended up using it.

Does your mystery valve have a black rubber button on it?

[dmorg1]

Is the valve you are referring to?

If yes, then it is a Hold Off Valve that forces the rear drums to actuate first.

[Simple Man]

Originally Posted by dmorg1

Is the valve you are referring to?

If yes, then it is a Hold Off Valve that forces the rear drums to actuate first.

Yeah, that's the one.Never knew what it was for.

I don't see fluid on the booster, I would think the rear reservoir would empty first, if it was leaking from the M/C. Rear reservoir is fine.
The pic of the AC Delco M/C on Rock Auto shows something different than mine. My M/C has the 2 bales to hold the cap down, the one pictured has a bolt in the center, to hold the cap on.