When doing a rear disc conversion do you need a master that is specifically for a disc/disc setup, or is just because you need a bigger piston size? I already have a 1 1/8 master, but I believe its for a disc/drum setup.

Drum brakes have a 10lb residual pressure valve in the master cylinder. The valve looks like a strainer. On my '62 Corvette (single master), I remove the valve when I installed the front disk brakes.

So, you should be able to remove the residual valve from the master cylinder and you'll be fine. If you do not remove it, the rear brakes will be always locked...

Theo.

I thought the disc/disc proportioning valve is suppose to take care of the brake pressure issue and what not?

Drum brakes need to have pressure applied on them all the time. Disc brakes do not.
The proportioning valve is for distributing pressure when you apply the brakes.

You still need to remove any drum brake residual pressure valves from the system for the disc brakes to work.

Theo.

I didn't think they put residual pressure valves in the master.

From reading info on Master Power's web site, they say it is in the combo valve (along with the disc-delay and proportioning valve).

Maybe they were only used on older models or 4 wheel drum masters which didn't have a combo valve.

Anyone know how to tell if a master has a residual press built in without opening it up?

Well... this is what I know:

Single master cylinders with dual drum brakes have the residual valve on the master cylinder.

On my '68 Chevelle, dual drum & dual master, I am about to add dual disc brakes. When I asked MP Brakes about what to do with the factory combination valve, they said to leave it there. I only have to replave the master cylinder. This makes me believe that there are no residual valves in the factory combination valves on dual drum brake cars... I may be wrong but I would look in the master cylinder first.

Another way to find out, in addition to calling MP Brakes, is to look are a master cylinder rebuilt kit for your car. If the strainer looking things are there, there is your answer...

Good luck,
Theo.

They told you to leave the factory valve?? Is this the same as a proportioning valve, cause MP brakes sells those valves for disc/drum applications and disc/disc applications. So how can they tell you to leave the stock one?

Here is what I know which isn't much. I have a friend that has a 62 Corvette that he races in vintage events. Some previous owner installed 4 wheel manual disc brakes on the car and a disc drum dual master cylinder. He always had a problem with the rear brakes retaining some residual pressure and you couldn't move the car by hand because of the rear discs. I changed the MC to a 68 Corvette manual brake disc/disc unit and no more problem. The car didn't have any factory style combination valve on it. Just my experience. GL

Stroked66,

My Chevelle came with drum brakes all around. There is a combination valve (or junction box) on the frame below the master cylinder. I asked MP Brakes if I should remove it when I install their disc/disc system. They said that I should leave it alone.

So, this leads me to believe that the residual valve on the stock system is in the stock master cylinder.

I will be replacing the stock master cylinder with the MP master and their combination valve. It seems that MP Brakes incorporate any residual valves in their conbination valve. But, I believe, that's not the case with the stock drum/drum system.

Again, that's what I determined. Your mileage may vary, as they say...

Theo.