I'm trying to fix my braking problems which I've been told my prop valve is the problem. I bought a new wilwood adjustable valve because I plan to upgrade to rear discs soon anyway.

First, tell me if I got this right. The thing hanging on from the Master cylinder is my proportioning valve, and the thing on the frame is just a distribution block. Only the FRONT brakes go through the prop valve. Right???

So I remove the old prop valve and replace with the new one, still only controlling the FRONT brakes???

The front (disc) brakes have a small line, and the rears (drum) have the big line. When I change to rear discs I won't have to change any of this, will I?

Thanks in advance!

I'm not sure about the factory setup. My 68 drum/drum car only had one device in the brake lines, it was on the frame. I believe it was a combo prop valve and dist block.

If you are gonna eliminate all this factory stuff(and I think you should if you want adjustable proportioning)put the wilwood valve in the rear brake lines. It reduces line pressure, and you want full pressure to the fronts, and less to the rears. The amount less depends on rear brake size and type. Fro drums you definately want to reduce the rears, I don';t know for sure about rear discs.

Thats the setup i have, an adjustable on the rear drums, and a line lock on the front discs. I split the front lines with a "T" fitting. Simple as that! If you add the adjustable one in the front lines, you are sure to have less than optimal braking performance. If you keep the thing on the frame, make sure its not another prop valve.......

good luck smile.gif

chris

The factory part that I believe is the prop valve hangs on the side of the master cylinder, and only has the front line through it.

The distribution block is a little rectangular block with 2 "in" and 3 "out" lines and an electric wire going to the "BRAKE" light on the dash.

It doesn't make sense to me why the prop valve would be on the front line though??? but that's the way the factory setup appears to have been. The brakes worked fine for the last 2 years, but now I can't get them working right after having the master cyl off and trying to re-bleed them.

The factory valve located just under the master cylinder is a "hold off valve", not a proportioning valve. In the stock disc/drum application, the hold off valve prevents pressure from going to the front brakes until the back drums have received enough pressure to overcome the springs. Once a little pressure is built in the back brakes, then the hold off valve allows the fronts receive pressure; the idea is that the fronts and rears will apply at the same time.

If you want to add an aftermarket proportioning valve (which will allow you to reduce pressure going to the rear brakes), you need to add it in the line AFTER the distribution block on the frame (i.e. the line going along the d/s frame rail.) Since the aftermarket valve will reduce pressure, it needs to be after the distrubition block. If you install it before the block (between the master cylinder and the distribution block), the distribution block will see a difference in pressure between the front and rear ports, which will trip the internal switch cutting off the rear brakes and turning on the "BRAKE" light on your dash.

What is your current braking problem by the way?

Either way, you will have to eliminate the factory hold-off valve when you install rear discs. Removing it now just means the rear drums won't kick in as fast as the fronts. When I had my car with a stock disc/drum setup I ran around for many months without the hold off valve, and never had a problem.

Troy

Troy, Thanks!

Its starting to make sense now, but can I keep the distribution block AND the proportioning valve? It seems to me that if that thing on the master cyl isn't a proportioning valve, then the distribution block on the frame would have to act as a proportioner. :confused:

The problem is that I can't get my brakes to bleed right, and a mechanic told me it's probably the prop valve causing this. It sat for a week with all 4 bleeders open, and not a drop came out, even after buying SPEED BLEEDERS, I still couldn't get anything to the back. I'm thinking now that the master cylinder is the problem though, because when I cracked the lines loose from the master cyl, fluid came from all 4. Since I already bought the prop valve, I'm going to use it.

What would happen if I put the 4 wheel disc master cylinder on now and kept the drums for a little while??? could I proportion it correctly with the adjustable valve so they'd work OK???? I don't want to buy a master cylinder now, and another one soon when I upgrade.

go to Master Power Brake site, mpbrakes.com I think it is. It has a frequently asked question board, gives lots of great info. Read that first. Concerning your bleeding problem, I read somewhere that on the combination valve on the frame, there is a button that should be held down when bleeding. I saw it on one. Maybe I read that on the MP brake site. Look at your valve and see if there is a button like think at the front that can be depressed. Hold it down with a clamp and the brakes should bleed some by gravity.