I inherited this problem so I’m not sure about how it worked in the past. The car was a manual drum and has been converted to front disc with 4 piston Stainless Steel Brakes Corp conversion. I removed the leaking master from their kit and installed a new one from the parts store for a 70 Challenger. It has the correct looking oem 70 booster. With the way the rest of this car has been cobbled together, it may have the wrong linkage or the booster could be from a power drum e body. Is there a difference? I forgot to look in my parts book last night.

Now it has a really low pedal. You have to lift it up with your toe to get it all the way back up. The front brakes are pretty touchy and work good. After adjusting the rear brakes where they will work now, I have constant brake pressure. On the rack, you can’t even turn the tires by hand on front or rear until you lift the pedal with your foot. Does the booster help return the pedal back to the top? It has pressure to all corners and it bleeds well. It has a correct looking distribution block on it but it must be a reproduction. It has all new lines. My first idea is looking at the master and comparing the linkage with my factory AAR.
Any other ideas?

Sounds like a bad booster to me.

The only bad booster I’ve ever had on anything had no vacuum assist so it was hard to push down. Does the booster help return the pedal to the top?

The power brake linkage has an extra bellcrank and less leverage vs the manual setup. If you still have the manual brake linkage that could be a problem. Pics in the attached link may help.


https://www.mpbrakes.com/booster-master-...0AAA06C0EDE55ED

I have the power brake pivot for sure but I’m not sure if it is 70-71 e body. They’re subtle differences between them and 72 up e body and all b bodies. Your picture is of the aftermarket setup that has all the mechanical linkage forward of the firewall. Mine is inside the firewall just like factory.

I'm kinda winging it here but I would split the MC & see what the round nub pushrod clearance to the bottom of the rear piston "thimble" is & you would (also) see if the piston is all the way back against the stop/snap ring (highly likely it is) & as said continue to checkout the linkage. You might press the pedal (idling) with the MC seperated & see if it (booster/linkage) now acts "normal". This might help you pin it down. Holler how it turns out.

To answer your question, yes the booster along with the master cylinder should push the pedal back. Did you say you do have the bellcrank? The Ram Man has a video on youtube about trouble shooting boosters. One last thing after you get it to working and releasing, the pedal should be close to the floor, about the same as the gas pedal. That is the way power brakes are supposed to be, but not bottoming out on the floor.

Had a similar problem, except my brake pedal would slowly sink to the floor at a stop... I was blaming the booster on losing vacuum until I had a long talk with Booster Dewey... About all the booster does is supply additional pressure to apply the brakes.. It really does not interfere with or create what you are dealing with..

Of the MC's I have rebuilt, the spring inside it should push it back.. Take it apart and look it over.. Worse scenario is maybe putting a kit in it.. Mine works great now and it was a rebuilt I purchase in the past... I wanted to keep the "correct" bolt down top MC, is why I rebuilt mine instead of new.....

If you take the booster out of the car and supply vacuum, then push in the rod like applying the brakes, and let go, it should return on it own, and along with the spring in the master that returns the pedal.

it does sound like a booster issue.
when you say the old mc leaked, how did it leak?
I had heard of a booster full of brake fluid before.

IF it worked before other than the mc being bad, changing just the master should not affect the way the pedal worked.

But there are a few tests you can run.

First with the car off, push the pedal down 3 or 4 times. It should remove all the assist.

If the pedal returns after all the assist is gone, then it is the booster.
IF this still doesn't return, then leave the lines hooked up to the master cylinder, but unbolt it from the booster as Robert sugguested.

But this time, press the pedal down with the car off and see if it returns on its own. If it does, then something weird with the master. If it still doesn't, then I would say something wrong with the booster or as others suspect possibly the linkage.

But as I said, I think you will find after the assist is removed that it will return as expected.

I had a booster that sucked the pedal down, to where it was applying the brakes slowly as I drove. Didn't realize it till I found my foot flat on the floor accelerating trying to keep it at 40mph.
I unplugged the vacuum to the booster to get home.

I did check the clearance of the ball to the thimble on the master and set it to the same spec as my old oem takeoff master and booster for a parted out 70 challenger with power disc. I’m on my way to the shop to do some more checking. I’ll let y’all know. Thanks for the help. The pedal was low the only time I drove the car before I started the rebuild. They had a vacuum pump installed on a sensor and relay before because of low vacuum with a big cam. The brakes were way touchy and the pedal was low like now. But they did work. I’m suspecting this is a ongoing problem. The old master did leak out the piston hole and down the booster enough to ruin the paint on the frame.

Ok. Here’s an update. I looked closely at the bell crank linkage and noticed some play in the pedal when you pull it up by hand. It looks like they may have put the system together and left off one or more of the bushings in the linkage. The 1/4” of play amounts to about 1/2” of travel before the booster moves to activate the master. I separated the booster from the master to see this. I also noticed that the studs that attach the master cylinder have been changed. They are longer and don’t have the taper. Also there is a new looking rubber part around the plunger and the rod looks nice and shiny. I’m thinking the booster has been rebuilt. It has the same patent pending numbers as my spare booster but someone put body filler over the pentastar. Go figure. It’s probaly a parts store reman.

I slightly separated the master and had my friend push the pedal. If you take up the slack first, the rod moves as soon as you push the pedal. It occurred to me that I have not had rear brakes with the old master. You had to push it almost to the floor and the front brakes would grab instantly. It was the same way with the new master until I adjusted the rear brakes shoes real close to the drums. Then they were so close, they would not release good. My front disc would release after a few seconds, but my rear never would. Especially after driving and getting hot enough to expand against the shoes. So now I pull the master off to check it. The first thing I check is for the check valve in the front reservoir. It is not present in the latest reman. I checked the one that was on the car that probably came with the SSBC disc conversion and it did not have one either! I pulled out my NOS 70 ebody to show my friend what I was talking about. You think these guys would know the drum brakes require this to work properly. That is why I had to push the pedal down to the floor to move enough fluid to engage the rear brakes. By then the fronts were taking all the pressure and grabbing hard. When I adjusted the rears to work at all, they were too tight.

So looks like I’m in for under the dash session with that easy-to-reach brake pivot. Lol. I will also have to explain to a parts man at O’riellys why his master will not work on my car. I’m ready for a blank stare on that one.

Has anyone had luck finding a new or rebuilt master for a 70 ebody at any local parts store? I need to check it for the residual valve before I take it home. And I know what you’re thinking. The answer is NO. I’m not using my NOS master on a customers car.

A M/C rebuild kit is cheap at RockAuto and then you know it would be done right, the disc kit includes the RPV.

As Forest said, with these kits, " you never know what you're gonna get"

If I am reading this right, the neoprene nub that is on the end of a manual brake pushrod to lock it into the "thimble" is there on your setup & I was thinking it ain't needed (or wanted) on a PB app (not sure tho) and I was under the impression that later (OE at least) wheel cyls had a thin flat dime sized metal plate between the cup/piston to help it keep its shape to avoid the intro of air into the line & this replaced the check valve in the drum half of the MC. As you noted I would take care of the bushing/slop under the dash & get that squared away. Holler how it goes. EDIT I'm thinking when you get the bushing clearance dealt with & bleed em out that you might be OK on this deal. post how it goes.

You can still get new m/c at auto parts stores for disc/drum setups, Rockauto has them too.

They all list the disc/drum master cly but none have the check valve. I went in today and was talking to the store manager and she was clueless. Up walked up one of the employees that was a friend of mine from way back. He is an engine builder delux. I ask him if he knew the difference between a drum master and a disc master and he started to explain the residual valve to the manager. She was dumbfounded. I ended up ordering two and see how they come in. I bet they won’t have one.

Someone is reproducing those nylon bushings for your belcrank hdwe.. Forgive me, I cant remember who, but I saw it somewhere recently.. I remember the ad well as I treated my old ones with kid gloves when I did mine as I was pretty sure they were unobtainable then....

Hopefully some one will chime in with who has them......

I think this one is missing the big steel bushing. It has lots of slack.

Well, I hope I got to the final problem with this car. Idiots should not be allowed to work on cars. The booster is a 70 for sure. It requires the large bushing with o ring in the eye of the booster. The problem is the “ mechanic” that did all this work used a 71 pivot assembly. The 71 setup does not use the large bushings only pins. The pin measures .372. The hole in the 70 booster is .570. They used the booster and did not have a bushing and they did not even use the correct size pin. They used a regular 1/4x 20 bolt with a lock nut. They could not tighten it because the 70 booster eye is so large they had to pry open the pivot to get it to fit the eye. Of coarse it had play on the booster and the pivot. About 1/4 play! That play with no residual valve made for a really low pedal. I pulled a residual valve from and old drum brake master cly and it fit right in. I’ll bleed the brakes on Saturday and we should be ready to go. Or at least on to the next problem these idiot created. I think I have found them all now, though.