I have a rear disc setup on my 70. The problem I am having is the rear calipers gripping the discs pretty darn hard all the time. The rear wheels get too hot to touch from a short drive. Not sure how long this has been going on.
I located a problem on the drivers rear which was an improperly aligned caliper bracket that was causing a conflict. I bought a new one and replaced it & put in new pads. The other side has no 'obvious' mechanical issue. But I still have the problem.
I have a wilwood prop valve that doesn't help this situation turned full either way, or turned to the middle. (before someone asks, the rear end has fluid ;) ).
The front discs work fine, and don't get hot at all. finger touch after a drive, the rears would sear skin.

Now I've been asked about the master cylinder before, but if it is correct (I honestly don't remember if I switched it for the disc brakes, or even the order I put the brakes on (new front discs first or rears first..)) I am not quite sure how having the wrong cylinder would apply pressure to the rears all the time.
The car is still in storage for a few months maybe less depending on the weather, but I want to get a plan of attack for this problem.
there doesn't appear to be something "lodged" in the line for the rears as I can still pump fluid out of the bleeder valve on both sides. So I'm scratching my head.
the drums worked fine, if I didn't have these wide-by-large wheels I'd go back to them graemlins/clonk.gif

matt

I know that when I asked about which master for 4 wheel discs and was told to get the 68 corvette one. It does not look like the one you have....

the M/C for rear drum cars has a "residual pressure valve" in it. it keeps a few pounds pressure in the rear brake line for faster take-up from the drum brakes. If you use this type M/C in in a rear disc car you're going to have exactly the problem you're descibing. The little valve is often called the "salt'n'pepper shaker" for it's appearance. We used to remove this valve from he plunger assembly in the M/C on race cars that had rear disc. It's been so long I can't remember the exact procedure but if you have a spare M/C you could tear it apart and see. Or, if you're brave you could just tear down the one you have and do it. I recall that it was a trivial procedure. The other fix if you have the $$$ is the corvette M/C, but I seem to recall that there were some other complications installing it. You could buy an overhaul kit for your current M/C and fix it, then pop it in and go.

be assured you'll fry your stuff if you continue to run it as is.

Tom

Originally posted by Tom Mobley:
The other fix if you have the $$$ is the corvette M/C, but I seem to recall that there were some other complications installing it.The '68 corvette NON-POWER (not sure why non-power, but that's what I was told, and it works) master cyl, bolted directly on to my stock Chevelle Power booster, and works great. I think it ran me about $30-40 at the parts store, +$10 because I had no core.

I've got a MC with a snapped ear on it (dropped...). Tom if I dig through that and remove this "shaker" will the MC work correctly for my rear drums, or should I spend the 40-50 on the '68 vette non-power as derek mentioned?
If this turns out to be as simple as the master cylinder, that would be great, although it is something I should have noticed earlier.

I can go either way, but I have a dropped cylinder to play around with if I should go fishing to see where it is and if I can get it out w/o damaging anything.

thanks!
matt

You need a Napa Auto Parts Master cylinder Part #39018 with a 4 wheel disc proportioning valve.

I hate to sound like a twit but you lost me on the 4 wheel disc prop valve. Do you mean the big brass distribution block near the front or the wilwood adjustable prop valve (came with the rear discs) that I have? I do vividly remember what a pain flaring the SS line was putting that prop valve in.
there is a napa right in town so that master cylinder should be easy..

Thanks..

Derek's solution sounds the best, especially if the $30-40 part still holds. You can turn in your Chevelle master cylinder as a core later. smile.gif
i thought I remembered there was some deal with the fittings not being the same size or needing a odd pushrod from the pedal, but Derek reports otherwise. If you want cheap it out take the plunger out of the broken M/C, pull the RPV off it and put it in your existing M/C. I'm actually not real sure you have to have the disc/disc prop valve, I don't recall ever having an issue using the stocker.

Tom

I just did my swap last August, so the price should still be pretty close.

I have no distribution block, just a Wilwood adjustable prop valve on the rear line. (took a few fittings and adapters to get it plumbed up though)

I'll dig through my receipts.

OK, no luck on the reciepts redface.gif but I did find my missing timeslips :cool:

Thanks guys. I'll take a look at it as soon as the weather gets better.

Matt

In my chassis service manual for my 69 cutlass they picture and describe the removal of the little residual pressure valves from the master cylinder. I have been told that around 1972 they stopped putting those residual valves inside the master cylinders because the function was taken over by the "combination valve" on the frame or under the master. If you could find a chassis service manual for 69 or so you could see the pictures and description I speak of. Good luck.