Disc brakes on Dana 25 with stock single chamber MC

Hello all, I've done a ton of research on the disc brake conversion and found a ton of posts here with a wealth of information, but none of it necessarily pertains to using the factory single chamber master cylinder. I have a 1961 Willys 4x4 station wagon with the Dana 25 front and have installed the 1/2 ton GM disc brakes.




After test driving it I realized that the front brakes were dragging and found that the master cylinder has a residual pressure valve for the drum brakes that needs to be removed.

There are a few issues with installing this residual pressure valve while still using the original single chamber master cylinder. I don't want to go with a dual chamber master cylinder because that would require some sort of adapter bracketry and/or pedal modifications. Keeping this wagon as original looking as possible is a big concern to me.

Because there is only a single residual pressure valve in this single chamber master cylinder, when I remove it that removes the residual pressure for both the front disc brakes as well as the rear drums. What I plan on doing is running an inline 2lb. residual pressure valve to the front brakes, and a 10lb. residual pressure valve to the rear drums. Is it necessary to use a residual pressure valve for the rear drums in my application (single MC)? My concern is that with a single chamber MC and a 10lb residual pressure valve to the drums is that they will be doing most of the stopping work since the front disc brakes require much more pressure from the same reservoir/chamber.

I'm going to need to redo at least the front brake lines because one outlet from the MC goes to the passenger side front drum and the 2 rear drums; while the other outlet from the MC goes to the driver side front drum. This is how it was from the factory, doesn't make much sense to me but that's just how they did it.

What are you guys' thoughts on this setup with the factory single chamber MC? Has anyone ever run disc fronts with this MC?

Thanks for your help in advance, this is really the only thing left that has kept me scratching my head. I've rebuilt the entire drivetrain so the only thing left to do is the brakes and then it's on to body work and making it pretty.

 

You will need the 10 lb. residual pressure valve for the rear drum brakes. It obviously holds pressure on the rear hydraulic system and this keeps the lips of the wheel cylinder seals held against the bore of the wheel cylinder to prevent leakage and bleed down. Without it you may have serious issues.
The front you will need the 2 lb. to hold a slight pressure in the caliper. If you don't the caliper piston can retract too far then you will have to pump the brakes to get the pistons to move out far enough for the pads to contact the rotors.
I too have the under floor single chamber master cylinder. I did not do any of this and have very few brake issues but I'm running 4 wheel discs so the scenario is a little different. I don't have the brake dragging issue that many people have but I suspect this is because the residual pressure valve in my mc is weak. I will eventually be changing this out and will add the correct residual pressure valves in the future and remove the one from the master cylinder.
Also make sure you use organic pads with this set up. Semi Metallics and ceramics are designed to be used with a booster and are much too hard to use with a manual m/c. In other words you won't stop worth beans.
If you have issues with the rear brakes locking up you can easily add an adjustable proportioning valve that will limit the line pressure to the rear brakes. This will not eliminate the need for the residual pressure valve however.

I've done this conversion many times and I'd suggest doing it by the numbers otherwise you may pull your hair out in frustration trying to sort out your brake issues.

 

Thanks, that's just what I needed to hear (someone else using the single chamber MC). I'm going to go ahead and order the inline residual pressure valves, make a new front brake line, and give it a world again. My master cylinder is in great shape (must have been replaced at one point) so I'm sure my check valve is working pretty good too. I didn't even go half a mile around the block and when I made it back I could smell the brakes...

 

How about adding rear disks to solve your problem?

 

So I just put a new MC in my '67. It is the single chamber MC. I have a Dana 44 with discs in the front of my Jeep, and the factory drums out back. I have bled my brakes FOREVER, and get no good line pressure. I have to pump the brakes once, then the braking is sweet, and works normal for about 15-20 seconds, then if the brakes aren't used again during that peroid, I'll have to pump the brakes once again to initiate good braking the next time I brake again. So, is this the same problem? I need to plump a 2# residual pressure valve into the line going to the front brakes, and a 10# residual pressure valve into the line going to the rear brakes? Then MAYBE install a proportioning valve if the braking is just a bit off after that? Do I have to do anything else with the master cylinder itself? Take anything out or put anything in? Or do I just plumb those two resid. press. valves into the front and rear lines and then my brakes should be good to go after ANOTHER re-bleed ?

 

Before doing anything else make sure your rear drum brakes are adjusted correctly. If out of adjustment it can cause the symptoms you describe and can make it difficult to impossible to get all the air out of the system.

 

Interesting, so I guess I need to check out the adjustments on my rear drums, then. Everything used to work fine, then a brake hose broke at the fitting, had to replace it, had to replace the MC, and now I'm having these issues. Didn't think to check the rear drums since everything used to be okay. Guess I need to check that before getting the residual pressure valves for front and rear, huh? And the MC was bench bled, and I've bled over a bottle of fluid through the system now, still have to pump the brakes once before braking is good.

 

whats your line sizes? I know that the driver front had a slightly larger line going to it, while the other 3 had a slightly smaller. the theory went that with the smaller line the pressure would build up quicker, and for the driver front so it wouldn't lock up first was a slightly larger line. That was for drum brakes though. might or might not also affect front discs in the same way.

A proportioning valve might help?

 

Not sure exactly what my line sizes are, but the brakes worked well before the one line broke, and I replaced it with the same size line, and did the MC at the same time. Now I need to pump the brakes once before the brakes will grab. They just barely take hold on the first push, and the pedal goes to the floor, but the jeep will stop after quite a while, so need to pump once then they work sweet. Guess I need to check out my rear drums before doing the residual proportioning valves, huh?

 

You say you installed a new MC, did you remove the check valve? I found that the check valve in mine is actually internal so you have to pull the MC apart and it is on the output side of the end of the spring inside the MC.

My brake lines are just as you describe. I ordered a new Y-fitting but I'm going to be making new lines anyways so I'm just going to have to see how it works. It is strange how they have one output going to 1 wheel and the other going to 3.

 

So am I supposed to get rid of the check valve that's inside the MC for my front discs/rear drums to work correctly?

 

Well, your MC should have the check valve in it and it should cause those disc brakes to drag because the calipers don't release all the way, in which case you would need to remove the check valve from the MC.

 

I'm not sure if the mechanic left the check valve in or not. However, I have the tires off the front of the Jeep right now and I can grab the hub and rotate the outer hub assembly. The pads do rub the rotors, but it isn't hard to turn. Are the pads not supposed to touch the rotors? I thought that how it is set up is correct. Are you saying that if the check valve was in then the disc pads would rub the rotors so much that I couldn't turn them by hand, or that it would be doing what it is right now if the valve was in? Also, lets assume the valve is in right now, and the brakes act how they do, wouldn't removing the valve mean I'd have to pump the brakes even more before they worked properly, instead of fixing my problem? I'm going to try to get the rear in the air tomorrow or the next day and see if the rear drums need to be adjusted out.

 

If you can rotate the front hubs/discs and rotate them by hand then they should be fine, as long as they don't heat up to an extreme and you don't smell them burning when you drive it.

With the check valve in (like in mine), when I have the brakes bled they drag so badly that I cannot turn them by hand without significant effort. Once I open the bleeder valve and relieve the pressure they're fine.

If your brakes are working fine, which it seems like they are, honestly I wouldn't worry about anything. I was just thinking that if the master cylinder had been replaced with an OEM replacement it should have the check valve in it, but your mechanic may have removed it.

As far as having to pump the brakes, I'd make sure your drums were adjusted as recommended above. If that still doesn't do it then I'd add an in-line Wildwood 2lb residual pressure valve for your front disc brakes.

http://summitracing.com/parts/WIL-260-1874/

I just received my residual pressure valves, so after I finish a few other things it's time to finish up my brake setup.

 

SiLicon, are you going to put a 10# resid. pressure valve in for your rear drums, or are you running discs front and rear? I gotta get the rear in the air and try to adjust the rear drums, then maybe get some of these valves! Good luck to you on your brake setup. My front discs don't heat up, can never smell them, they don't drag that badly, I just have too much pedal travel!!

 

Yes, I bought a 10lb in the rear for the drums, and 2lb in the front for the discs.

 

iirc, discs usually work best with smaller lines thus higher pressure. The factory Jeep lines are probably too big for the factory MC.

I'm a big fan of Wilwood universal MC's which a used on circle track cars. I think it has a 1" piston. I use a single with drums which I wouldnt recommend for street safety. They have a nice dual unit now which is what i will eventually switch to.