What MC w/ rear disc conversion?

Working on a 71 E-body for a guy; has a rear disc brake swap on an 8 3/4. It has the GM Eldorado calipers it appears, the ones with the parking brake cable built into them. On the front, has 11" single piston discs.

Brakes are working, but poorly. First half of pedal nothing really, last have it brakes but just not as good as it should.

I looked it over, does NOT have a prop valve on it; and it appears to have a stock rear drum front disc master on it.

Anybody know what MC should work with this combo? Will the stock one work with a prop valve? Should I add adjustable prop valve from like Summit on the rear line to fine tune the setup? (I run one on my 1st gen Cummins w/ a disc swap; and on my Dart with good results).

Thanks as always.

I believe with 4 wheel discs you need a large bore MC cuz of the amt of vol needed to fill 4 calipers but first: they ain't pumping up? pedal effort doesn't seem to be doing anything at the corners? all 4 corners swapped to discs at the same time? and poor performance since then/day 1?

I drove the car - only to probably 20 and hit the brakes a couple times. This car was "built" for a local car dealership owner that has some nice vehicles in his collection - the guy that asked me to look at it bought it once the dealership owner lost interest with finishing the car. Thinking it needed minor things like trim, etc. So this car has NOT been up and driving yet. Is a nice car, workmanship looks good.

Pedal is ok... not terrible but not as good as my Dart which is a power brake Kelsey Hayes car. It has a lot of travel before you get braking, doesn't feel like air, just feels like it's pushing a bunch of fluid before anything happens maybe? All 4 corners are working, can tell by looking at the rotors.

Suggestions on an MC? Big bore plastic reservoir one maybe?

I'm hesitant cuz I have no first hand exp with 4 wheel discs (yet,got several planned) just what I read here. I did have a 1&1/8 alum dual bowl (w plastic reservoir) on a 65 dart with '76 A (2.75") calipers up front (ceramic iirc pads) & 10" drums on the rear with the OE drum splitter which means no proportioning & it stopped flawless but that was slow everyday driving around town & it would lockup the rears first if I hit em hard. A mismatch on paper but zero issues & no detectable pad wear either. I'd CONFIRM there's no air in the MC (first) or the corners (calipers can be tough to air out completely) then get the biggest bore MC I could find

just looking in my wagner brake master catalog at a 97 chrysler with 4 wheel discs shows a 15/16" dia. master bore with 60mm [2.364 dia.] front calipers and 34mm[ 1.3396 dia.] rear caliper bores. i'm not sure on the rotor size or the car weight, but that is with abs and 16" wheels. also not sure if this info is relevant in this discussion or not,just something to ponder.

Power or manual brakes? Also, agree the prop valve may help, especially if the rear is locking first. Can you push it hard enough to find out? Also would not hurt at all to bleed the brakes just to make sure air is not spoiling the fun.

It's a power brake car (with enough vacuum to run the booster).

The more I've looked around I'm pretty convinced I'm going to recommend switching to a "new" style 2 bolt 1 1/32" bore MC. From Dr. Diff's site this is the one recommended for 4 wheel discs. Then add an adjustable prop valve. I can't for see that combo not working.

I looked up the caliper bore sizes and they are pretty big - I just don't think it's pushing enough fluid quickly enough; and odds are it's a smaller bore MC right now on there.

Sounds good. I run Dr Diff's 2 bolt master and works well. I have small piston brakes and used the 15/16" size. The bigger one sounds right for you. When I switched to discs on all 4 corners, I didn't need the prop valve though each situation varies.

Read up on those GM Eldorado rear calipers. I vaguely remember something about having to use the emergency brake to adjust the caliper piston because it doesn't react like a normal caliper. I took one of those buggers apart once to find there is a weird looking ratcheting screw arrangement. Reassembling the thing was extremely difficult. I'm still suffering from nightmares over the whole misadventure!

So the bad news is that you can't just mix and match brake parts and expect them to work. The front pistons size needs to be matched to the rear piston size in order for the brakes to be properly balanced. The correct ratio of front size to rear size is a bit complicated since it depends on vehicle weight distribution, how tall the center of gravity is, tire size, etc. but the usual rule of thumb is that you want the front brakes to do about 2/3 of the work.

I don't know much about the Eldorado brakes but my guess is that the pistons are too large and that you'll never get that setup to work properly.

I'd throw the GM stuff away and put a DoctorDiff rear disc kit on there. At least the DD rear kit is properly designed to work on a Mopar.

I have the eldorado calipers on the rear of my truck, and standard GM front calipers. I have a 1.25" MC, which I believe is too big (now that I finally got a rear caliper that actually works).

I think I'll have to go down to a 1 1/8 or so to get more pedal travel and higher pressure.


MAKE SURE that you can't see any light between the pads and the rotor in the rear. If you do, you'll lose most of your pedal travel to take up that gap.

And yes, the ebrake has to be used occasionally to keep the pads near the rotor.

That's true of any rear caliper that uses the caliper as an E brake as well, rather than a mini drum ebrake.

Just as an update, and to say thanks for the responses, I came to the conclusion this is a kit from Master Power Brakes. So I called their tech line - they agree that it needs a prop valve. So going to start with an adjustable one from Summit/Jegs and go from there.

Will also make sure the rear parking setup is properly adjusted.

I'm thinking the combination should make some difference.