Bottom line, use the right parts. Why would you use the 73+ spindles when the right stuff is available? There was an article in Mopar Muscle a while back, I no longer have the specs. They printed a chart with different variables between the earlier and later spindles and there were some differences, I think mostly with roll center. BUT, the test was not only skewed from the start with non-factory settings (lowered here, changed there, who knows if a stock setup would yield different results?), it also was very one dimensional - the factory ran these setups through simulations that measured everything seven ways from Sunday. One test will not find every gremlin, the same way that a standard alignment will not find issues beyond the basics.

The right parts are out there, use them.

"Sucess! (1) You'd think bad master but I'm not grasping how it was high and tight with the one (offending) side blocked off but spongy with it hookup up then a diff master made both front sides firm as you'd logically think there was still air in the bad side (2) To confirm (or deny) I'd get a diff one with a smaller bore and bleed it out since you're getting to be a bleeding pro and can do it in no time"

If I would block the fronts off completely, the pedal felt great. When I would add in one side of the fronts I would get a good pedal, but more travel when pressing the pedal. When I added in the other front side, the pedal would almost go to the floor. I initially interpreted it as air in the front system somewhere but now this makes me think it is a ratio problem with the levers in the pedal and booster. Too much stroke in the master? I don't know.

I will try a 15/16" or 1 1/32" bore master in the spring to try and get a better pedal feel. What I do know is this 7/8" bore does not work.

Maybe we got the answer further up, I’ve read all the replies but not all on the same day.

Regarding the bore size vs front brakes -
From my understanding, a small bore is for manual brakes, larger bores are for power brakes (to keep pedal effort within 'normal' for a human leg). There is probably also a slightly-larger bore suited for discs then, because I suspect that the fluid volume required to move 2 pistons is greater than to move 2 front wheel cyls. Also, the late-70s caliper bores are larger than the early 70s (at least as far as C-bodies go), which supports my hunch.

Using a small bore MC requires longer pedal travel/longer MC piston travel. There are also a few orifices/ports in there that I'm not fully versed with, but I do believe that if in the event of a pressure loss in teh front brakes (which run on the rearmost MC section) that the piston travels further forward and bump-stops the rear brake piston into action.

So climbing further out on the limb:
Small-bore MC takes lots of travel, can barely push the fluid vol to activate the calipers, but then physically pushes the piston for the rear brakes – and they lock up (especially because pedal effort is so low in this configuration).

Then one front brake is capped off, reducing the fluid vol requirements, so the pedal feels better, and blocking them both off makes a rock-hard pedal – proving the MC has integrity. The difference mentioned earlier, between blocking 1 caliper vs the other, is a mystery to me, though. Different length of metal tube was my initial thought but then discarded as the fluid vol required is same for both calipers and pressure loss in longer tube would be negligible.

And as a bit of evidence –
Almost 20 years ago I converted a 65 Fury from manual drums to power discs (c-bodies use a different pedal assembly for manual brakes, and you can’t tell the difference by looking down from the driver’s seat, you have to be aware of this difference). I used a 71 PDB booster /MC with my MB pedal. Braking was great but the pedal was very soft and felt like it had air in the lines. Bled them repeatedly, over several weeks, but to no avail. Blocked off the front hoses and got a rock-hard pedal, so I knew the MC was OK. None of the brake experts I spoke with had any good ideas. I didn’t drive the car very much, and the brakes were awesome, so I accepted it. I figured it out many years later as I learned more about C-bodies.

I think this is a related situation.

It doesn't, but that didn't stop anyone else from discussing it.

Sorry, should have clarified,

If I blocked one side of the front brake system ( didn't matter which side - after further tests ) I would have a decent pedal. When I hooked both sides into the distribution block is when I would get the overly long pedal travel.

I can't explain the results as fluid can't be compressed. How would the isolated rear brake system produce a great pedal, while adding the front lines would give me excessive travel? The signs point to trapped air but after switching masters, that theory is disproven.

I don't recall the actual bore size of the Ehrenberg Master but it is at least 1 1/8", possibly 1 1/4". ( Not 1 15/16" as I originally posted ). The Eberg master is what I have on the car now and is working but with an unnaturally hard pedal with only a little bit of travel.

UNBELIEVABLE!

The "system" is not working. Apparently my test drive the other night wasn't long enough. I was half way to work today when I noticed the car started feeling sluggish, parking brake not on, [Edited by Moparts - Family Friendly Site - Keep it clean]?
Seemed like every stop, it got worse and worse car didn't want to move as if the brakes were being applied. Unmistakeable scent of overheated brake material and the pedal travel got really short as well. I had to pull over and I loosened the fittings at the master for the front and rear lines and the car will roll freely. The front wheels were very hot, the rears seemed normal.

Apparently, I'm the poster child for what not to do in a disc brake swap.

Thats a symptom of a master cylinder push rod adjusted to far and its not letting the master cylinder plunger come back far enough....Fluid can only return to the master when it's all the way back.

I will check that before I leave for home. I thought I adjusted it even with the recess in the master but in the haste to mount the new master, the clearance may have been overlooked.

Do you have the brakelines swapped at the MC perhaps aswell?
The rear port is usually for the front brakes and viceversa.

And the port for the rear brakes might have a residual pressure valve in it, which would make the front brakes drag if F-R circuits are reversed.

MCs used to have the valve at the outlet seat, but supposedly they no longer need/have it due to expander cups in the wheel cylinder seals.

yes check the booster pushrod free play from the round nub to the bottom of the piston recess & want ~.060" or a bit less and take off the lines from the M/C and stick a drill bit deep into each cone and you'll feel the RPV if it has one as it's just a piece of neoprene on a spring & you'll feel the springiness. Cant have a OE type RPV on the disc side they hold too much pressure

Seems as though Hemirick was right. I adjusted the pushrod in a little and had no problems on the way home besides the usual stiff pedal.

Wow, did not know a sixteenth or eighth of an inch would make that much difference!

I do have a couple different masters coming in tomorrow to experiment with over the weekend to try and get a better pedal feel.

Looks like I finally hit on a master that gave me the results I was looking for. I tried a master from an 85 Diplomat with a vastly improved feel. The pedal travel seems natural and I can finally get the front tires to lock up before the rears.

Thanks to all for helping... so glad it is finally fixed. Now I get to put the car away for the winter