Car is a 70 Charger with factory front disc brakes and rear drums. It has the original under dash linkage and unrestored dual diaphragm booster. I started with a very good reproduction of the tall MC with engine side lines as it had originally. I’ve replaced it with an NOS MC 2944477 Bendix core 821 and they both have the same problem. Neither of them will actually lock up the brakes in a hard stop. The car stops but it feels like the MC bottoms out and does not push enough fluid to apply the brakes harder. I removed the MC and the booster rod moves 2.0” out of the booster without vacuum and about 2.20” with vacuum hooked up. With the pedal applied, you cannot push the rod back it to booster by hand. In addition, the pedal goes further down and you don’t feel it bottom out the amount of travel in both the reproduction and the NOS one is the same at 1.0”. Unless there is some kind of a problem with the booster that stops it when under load, I’m thinking it is bottoming out before it locks the brakes. I’ve changed all the brake hoses to oem rubber and I’ve bleed the lines multiple times. Any thoughts?

anything interfering with pedal movement?

No. When you take off the master cly and depress the pedal, it goes all the way to the floor with no interference. With the MC on it stops about 2” off the floor.

I looked in my 70 FSM, didn't see any spec for M/C stroke.

I would be tearing down the M/C and looking for issues as well as measuring the stroke.

is the pedal Hard or spongy? Can you see the hoses deflecting when STANDING on the pedal? I'm assuming the rears and the E brake are adjusted properly

Another thing you can try is with the engine off, pump the pedal several times to get rid of any vacuum in the booster. Then apply a steady pressure to the pedal and start the car, IF the booster is working properly the pedal should drop with the vacuum.

I’ve done that on two MC. The reproduction A1 Cardone number13-1475 and the NOS oem 2944477 are exactly the same. The secondary piston is about .020 shorter on the reproduction, but that does not make a difference. They both have a 1” stroke.

The pedal is not hard or overly spongy. I’ll check the rears again for proper adjustments. I’ll need help to check the hoses under pressure. I thought about that yesterday. I’ll try the procedure with the engine off.

Ok. Just did the engine off test and the pedal goes down when the engine starts but it still bottoms out and hit a hard stop before it gets to the floor. So it the same as before.

Mark, 2 inch travel seems like it would be enough. Have the meter valve and proportioning valve? You could measure the piston travel in the MC to bottom with the one you removed. Also there is a gap setting between rod and MC piston.

You bench bled the new m/c, right ??? Then bled all the brakes, right ?

Bled them several times.

what prompted the mc change or what caused you to do brake work?

have someone crack a bleeder open and see if the pedal moves further.
that will tell you if it is physically possible to move vs something in the way/stuck.

when you changed the brake hoses to "oem rubber", were these NOS or other new dealer leftover stock ?
perhaps the "oldness" of the rubber is causing issues internally ?
just wondering out loud here.

As previously mentioned, have you verified the clearance between the PB pushrod and piston inner bore. Should be ~ .020 -.030 clearance.
too much will limit MC output Too little will cause the brakes to drag.
It is something that should ALWAYS be checked when changing MC's or Boosters as things can vary between suppliers

I had the same issue on a friends 70 Coronet R/T. It was originally a drum brake car. Converted to factory pin type calipers. Never measured the caliper bores. But the issue was the system needed more fluid than the master cylinder could provide. This caused the pistons in the master to bottom out before delivering enough volume. Ended up going to a bigger bore master cylinder. Cured the problem.
Doug

I’ve made some discoveries but I’ll have to reply later. I’ve got to get to a meeting. Still not solved yet though.

DVW brought up an interesting point that I previously thought of but did not mention as you said they were NOS parts I think .

regardless it is always a good idea to Verify the piston sizes as parts can get swapped between boxes and with rebuilt or replacement parts things can vary. I've chased my tail on many brake issues.

one that always come to mind is a low pedal but working well on a 75 or so Corvette. Finally found an old GM parts guy on a Corvette forum that said somewhere around 74 or so, GM changed the power brake booster to M/C pushrod length but did not change the part # as they were only sold as a unit by GM. NICE!!!!!
Fast forward 45 years and you have the replacement parts industry totally confused as there are now booster and masters available individually that are listed for 68-80 IIRC but may or may not interchange.
I should mention the car came in we did an inspection and we wound up doing the brakes. We did not however measure and record the pedal height while doing the inspection's. Doing so was added to the list and we did from this point forward 🙄
One of the other clues was I had a 69 at the time and the brake pedal was noticeably higher and above the acc. pedal.
This by design was to prevent one from hitting the accelerator while braking

This fiasco cost about 40 hours

To address the brake lines, they are not NOS but new ones from inline tube. They did not flex when I had a Freind step on the brakes.

Also. I rebuilt the entire brakes system as part of a mechanical restoration on the car. It was parked since 1979 and just used the original booster, linkage, safety valve , proportioning combination valve. The mc had already been replace with an aftermarket one with the provision for the lines on either side and was stuck. So I did not want to use it.

I’ve got some time before I head to the shop and try something this morning so here’s what I’ve found. George( dragon slayer) sent me publication for a master tech service conference book for 1970 brakes system. It was very informative. I think it’s available at the ebodies.org site, too. I’ll post some pictures from my phone but I may have found one problem with the aftermarket master cly I was using at first. It’s Cardone unit 13-1475 with engine side outlets. They don’t sell it anymore. In reading about the original factory master cly, it explain the relationship of the pistons to a “ compensating port”. The cups on both primary and secondary pistons are set slightly exposing the port to the reservoir. It’s about .025”. The main function is to bleed air or excess pressure back into the reservoir when the piston is at the rear most position. So. It this adjustment that is critical on the installation procedure.

I think I will head to the shop and take some more pictures to explain this problem. The internet is full of this “ adjust the pushrod” instructions but this mc is a horse of a different color and pictures will explain better.

I have verified that my booster is probably not the problem even though it leaks slightly from around the pushrod seal. I had a short, rude conversation with a booster rebuilder who told me my problem was that I had not adjusted the pushrod and hung up. I’ll just conclude that he knows a lot of general information on brakes and boosters but this is an odd duck that even some mopar guys don’t understand. There is absolutely nothing in any service manual about adjusting the booster to the master cly on a disc brake application. The only reference is on a Midland- Ross drum brake setup and this is completely different. I’ll take pictures and explain later.

Mark, if you look in the service manual it does talk about seeing bubbles or swells coming out of the compensation port as you apply vacuum to the booster. If the pushrod is too long and seals of piston block that port pressure is not fully relieved, and brakes can stay on.
It does beg the question if the aftermarket used a smaller piston/bore so volume is less. Originals are 1 1/8" for the 67-70B. I do think getting a pressure reading is necessary to really figure out what is going on. The other thing that I thought about is the pads and any contamination that limits max holding.