cefolar

I am reaching out to the ever wise community for some help with the brakes on my '61 Resto-rod. They have been driving me absolutely nuts.

First it is a power disc conversion - with an 8" booster and standard 2 chamber master cylinder with a GM Disc/Disc proportioning valve. All parts are brand new (or as a friend says "untested"). I bench bled the master cylinder and adjusted the pushrod and everything seems to be mechanically fine. Here are my 2 issues:

#1 - can't get the rears bled. I have bled them several times and can't seem to get a hydraulic "push" to the rear circuit. When I crack the bleeders I get a slow stream of fluid and the pedal does not "drop". Front circuit bleeds fine, I get good hydraulic pressure and pedal drops when the bleeders are cracked. There are no visible leaks in the circuit from the M/C to the back brakes. My guess is that there is a leak somewhere (possibly just air) that is not letting the rear circuit pressurize. To that end....

#2 - to try and diagnose the problem I disconnected the rear circuit and plugged the outlet in the M/C. As suspected I get a good solid pedal to the front circuit....awesome. I took it for a short test drive around the block to see how just the fronts felt. GREAT - until I shut the car off. When I refired it to pull down the driveway I had a completely solid pedal and NO brakes at all until a couple quick pedal pumps and then they came in again.

I'll take any ideas I can get....I have read a bunch of the other threads and don't think it is the proportioning valve, but I'm willing to try anything at this point.

DansYellow66

This is from the left field - simply guessing crowd - but #2 sounds like the master cylinder is still not fully bleed of air or has faulty seals on the pistons (bad Master Cylinder).

wmf62

try bleeding from every connection from the m/c to the rear brakes and see if you get fluid.
Bill

GCD1962

some caliper bleed screws need to be positioned downward in order to bleed correctly. Can't be done bolted up in normal position otherwise air remains trapped.

The angle of the bleeder screw on some calipers do not allow all the air out when the caliper is bolted on. You need to take the caliper off and turn the whole caliper in a position that allows it to let all the air out. After its bled then bolt it back on.

GUSTO14

You mention that you are using a Disc/Disc proportioning valve. Is your car using only front disc's or is it 4-wheel discs?

Is this "proportioning valve" from a 4-wheel disc C2-C3 Corvette? Corvette 4-wheel discs are really proportioned mechanically by the use of different sized pistons in the front and rear calipers.

If it is from a C2-C3 Corvette, this is not really a proportioning valve, it is really a distribution block that has a valve in it to isolate the front/rear from the master cylinder if it detects a leak in the front or rear system. Normally this would be obvious because the valve would turn the brake light on. If the brake light is not connected in your car you might not be able to tell very easily except that one side of the valve would have little pressure directed to it.

If you are using a real proportioning valve such as this...

you may have it adjusted to not allow enough pressure to the rear brakes.

Good luck... GUSTO

MikeM

Good place to start. Right here.

cefolar

I converted to 4-wheel discs. I tried "resetting" by relieving pressure from the fronts and then moving to the backs (I have had success with that on other C3s), but didn't get anywhere with that. I am wondering if I should just scrap the prop. valve/dist. block all together? Or maybe move to a manual one like you have pictured. Do you think there would be a big issue not using a prop. valve at all?

I am getting fluid from the rear calipers, just not with much force. I was on the road of trying this by "capping" each connection one at a time from the M/C back until the pedal got spongy. I started by capping the rear M/C circuit and have a solid pedal. I was going to move to the dist. block rear connection next.

wmf62

FWIW, i have 4-wheel discs on my 62 and i don't use a proportioning valve and haven't had any problems.
Bill

MikeM

I don't think blocking the flow will tell you anything. Flow is what you're looking for. If you crack a line open and have no flow at some point, you've likely found the source of your problem.

mashinter

No brakes? I'd get rid of that master cylinder.

MikeM

Kinda' looking like a possibility.

Frankie the Fink

I have discs only in the front on my '61 (drums in rear) with an original M/C and NO proportioning valve. Car stops just fine.

cefolar

I think I'll start by scrapping the distribution block and go from there. Thanks a ton as always for the input.

MikeM

Any possibility you have that block plumbed wrong?

GUSTO14

If you can at least bypass the rear circuit of the distribution block by going directly from the master cylinder to the rear calipers it would tell you if the distribution block was the problem or point to the master cylinder.

As for the need of a distribution block, other than looking good, it's primary purpose is to locate a switch to inform you if there is a pressure differential between the front and rear circuit. Did you use a new distribution block? If not, it's possible the distribution block has the valve stuck in one direction. As you mentioned, typically if you relieve the pressure on one side or the other it will try to center the valve. But without a warning circuit to inform you it could be difficult to tell if the valve recenters.

Good luck... GUSTO