E body Power brake pedal problems?

Been working on friends 1970 Hemi Challenger repairing different items for over 20 years. He has had it for almost 40 years. 38,000 miles original car. Brings car over because it wont stop very well. I replaced front pads,calipers, and rotors. replaced rear wheel cylinders and shoes. When I went to bleed system,RF caliper wouldn't bleed. Found rubber hyd hose collapsed inside. Replaced all 3 rubber hoses. All wheels bleed fine. While in it, I suggest replace anything rubber. Bought new master cylinder and sent booster out to get rebuilt. When I open bleeders with helper pushing brake pedal, it goes all the way to floor. that's good. When done bleeding system it has high, hard pedal. But when road testing it, the car slowly stops even when standing hard on pedal. pedal goes down about and inch and is rock hard just like you but a 2x4 under the pedal. When put in reverse and standing hard on the pedal, car will slowly creep backward. So far everything in the brake system has been replaced except the 2 brass safety valves under the master cyl. I just ordered both new, will come next week. What am I missing?? Will the safety valves do something like that? Need ideas! thanks ahead, Jim

seems like you are not getting any assist from the booster.

I would verify it even does anything for the pedal first.
start the car press the brake pedal. see how easy it is and how far down it goes.
I would let it idle for a bit and try and see if it changes at all. Say 5 min or so depending on the vacuum that engine generates.

then shut off the car and pump the brakes 3 or 4 times. by the 5th it should be out of any stored vacuum.
see if it is any different.

if it isn't any different, or it is minor, I would then be checking the booster again. See if something was assmebled wrong.

Booster/check valve issue.

I believe it's 1-press pedal until it's "hard" 2-then while holding said pedal, start car. 3-pedal should drop maybe an inch or so.

Another item to check off your list, ....sounds like you haven't confirmed the booster to master pushrod clearance when you replaced the master... generally .020 to .060 clearance

Before I sent the booster in, I connected the booster only in my hand to a different vacume source from my daily driver. When I pushed on the rod with vacumme, I could push it easy. When they rebuilt the one I sent them, they said it wasn't bad, but we decieded that it has 50 year old rubber parts in it, so it was good to do. They sent a new check valve with it and I checked it by sucking and blowing on it. The hard pedal was there before I had the master cylinder replaced and booster rebuilt.

I did do that and the pedal did go down a bit after I started it. A weird thing I noticed is when I hold the brake pedal down hard and rev the engine, I could feel the pedal go up and down about 1/4" as I was varying the throddle. The vacume changes with throddle opening, but I never noticed that before.

This is another weird problem is with the new master cylinder and original booster and the rod screwed in to make it the shortest, I still had to put 4 flat washers on each booster stud to get a little clearance. So now the master is about 1/4" away from the face of the booster.

another MC might be in order & not sure if haveing the MC not sealed to the booster is a (vacuum) problem or not. & on the pushrod I would shoot for .020"

if it happened before you sent it in, then there is probably another issue.
the fact the master doesn't work without shims is concerning.

I would guess you don't generate enough vacuum. And you have the wrong master.

have you tested the vacuum that motor makes?

I just did the math on both MC and the bore depth for the pushrod is the same on both MC. The one on the car wasn't original, it was replaced with an aftermarket one. The new one I got was from NAPA. The original one had 3 thick paper gaskets between the MC and the booster and that is about the thickness of my 4 washers. The Hemi has 15" to 16" of vacume at idle and my Daily Driver has about 20"

also the rod adjustment on the booster is screwed all the way in to make the rod the shortest

that sounds low for vacuum.
also what is the part number on the master? you say it isn't original, but is it correct?

It is not a Bendex, The one on the car has no casting #. The new one from NAPA also has no #. All I know for sure is the Hemi Master has the lines facing the fender and they use a 1-1/8" bore, so that much is correct. I think I read that the correct Bendex MC with the correct # for a Hemi is the rarest and most expensive MC out there.

Does the piston protrude past the mounting surface of the new master cylinder?

If so, you must adjust the push rod so the piston becomes FLUSH with the master cylinder during assembly.

The piston is out about an 1/8". The screw in the bottom of the MC holds the piston from coming all the way out, but if I adjust it so the piston is flush, the piston will be forward of the brake fluid feed hole?

I'd put a cheap parts house rebuilt M/C on there just to diagnose. You likely wouldn't keep it on there but at least you'd know if it was the M/C or not for not much money. I usually dont shotgun parts at a problem but I (just me) think I would here on this one.

New master cylinders lack the sheet metal tab designed to hold the piston flush with the back of the casting.

If you adjust the push-rod so the piston is flush, the piston will be in the correct location.

There are 2 holes, one large and the small compensation hole. The seal lip should be just aft by .020" of the small compensation hole. The larger hole can still allow transfer of fluid when behind seal as it would push lip away from wall. The piston has to leave the compensation hole uncovered with brakes off, otherwise pressure does not bleed off. So it does sound like you have piston to far out the back and until you cover that smaller hole the pressure does not build up. Once seal covers small hole the piston are basically balanced and pressure starts to build.