Well, I bought one of those pressure gauges you screw onto the bleeder valve. They are easy to use. Screw them in and crack the valve to get the air out while help steps on the brakes slightly. Front brakes only had 650 to 800 lbs and developed very slow. It should have 1500 or so. It makes sense as to the way it stops when driving. I had to really push down the pedal all the way to get it that high. Back brakes seemed to work as they should. They would rise steadily until they got to 800 lbs. the booster works fine, I’ve tried a new reproduction master, one NOS, all new lines, rebuilt both metering and combination valve, all new wheel cly and rebuilt calipers. I’ve bled the brakes multiple times and get no air at all. Adjusted the pushrod. There is no interference under the dash. All I can think of now is start changing parts one at a time again. I guess start with the calipers. I just don’t see where the fluid is going when you strep on the pedal. The masters all bleed fine on the bench. I don’t get it. I’ve install many disc brake system with the new repop booster, master and valves and never had this problem.

there is a calculator feets has in one of his posts on what you should expect to see pressure wise based on the master and calipers.
I would ask, what kind of rubber lines did you use?
the build slowly part makes me wonder if they are giving some and absorbing some of the pressure and prevents you from maxing out as expected till you bottom out the master.

I'm not sure how easy this might be but, I'd check pressures upstream of the p valve, hold off etc.

Does the master have the oomph to do the job?

I'll repeat my suggestion to eliminate the factory monkey business and plumb the fronts straight to the master and put an adjustable p valve in the rear.

that is a better idea.
try at the master first and work your way down if you can to see where if any loss is.

i believe Feets calculator is a stickie in the handling forum.

Ok. So now here’s something to ponder. I drove my AAR to the shop to do a brake check just for grins. It has always stopped excellent. My buddy that drove it the other day commented how good they were. I tested ion the front. It’s has 700 psi and that’s it. Maybe the pads on the charger are not gripping? Idk.

My suggestion is to not take advice from someone that obviously doesn't know why there is a combination valve in the system.

To the OP:

I don't recall from your prior postings if you adjusted the rear brakes? Too much clearance between the shoes and the drum will cause your symptoms.

Food for thought. My racecar has a brake pressure gauge. It will hold at the line with 500 psi at 3400 rpm. You can get it over 1000 psi, but you have to really stand on it. It stops fine from 150 mph using around 300 psi.
Doug

In my 20 years of chasing issues like yours my experiences;
800 PSI is going to give you what you have. there are many variables to sizing, pressures, pedal ratios etc. but dealing with factory systems the following # is typical
1. it has been suggested to check the clearance between the end of the Booster pushrod and M/C Piston. Last I read you said it was not required. I dropped out at that time. Even with ALL FACTORY OEM parts there is a thing called TOLERANCE STACK UP. So, at this point I suggest doing so.
2. 1500+ PSI should be generated in a PANIC STOP situation. IE: Attempting to shove the pedal through the radiator. Some vehicles will generate 2200#
Note: holding the steering wheel with both hands before JABBING the pedal helps
3. Your comment on the pressure slowly increasing is not typical assuming the pedal pressure is constant.
4. Is it possible whomever is pushing the pedal is not able to apply enough pressure?
5. Providing the master is good and properly bled, Attaching the gauge to either port of the master will result in a rock hard pedal that barely moves, but generates above mentioned pressure.
6. One can plug the rear port and work to get the pressures up at the calipers. A plug can be purchased or easily made by folding a piece of tubing 2-3 times and then squeezing it in a vise or press.
Install the plug snugly. have someone put pressure on the pedal. Crack the plug to allow fluid and air to escape BUT CLOSE IT while the pedal/ fluid is still moving. Repeat 3 or so times.
NOTE: IF using the pedal pump method to bleed, I do this on both ports simultaneously when installing a new master to prevent air from being sucked back in the front port while the rears are being bled.
7. if you cannot get the pressure(s) up, start looking for things flexing or moving more than they should
8. Pads may help but IMO and experience you need to get the pressure up
keep us posted

How about this, could F/R lines be reversed somehow?

With the reported delayed pressure response, good question 🤔 But that delay would be controlled by the porp valve so I would think not, BUT definitely a good question and worth asking and for the OP to check. I'm not sure if the bore sizes Front to back are different in the master but if they are that could account for the problems.
I also believe the fitting sizes for the F/R ports are different to try and prevent this BUT???

All good thoughts here guys, thanks for the replies. The lines are not mixed up. I have the originals and the new ones are the same. I have other cars with the same valves and all the lines are in the same places. The back brakes are adjusted properly. I had them too tight once and it did not help. The backs work fine. They hit first as they should and then the fronts start to apply. Let me explain the adjustment of the pushrod first. I said it’s not the same as a master with a clip or stop to hold the piston assembly in the correct position, such as an ebody. The pushrod should be adjusted in those cases to have some clearance or almost touching. Although the service manual gives no such instructions. The 67-70 b body master that fits these disc brake car from the factory is different. It uses the adjustment of the booster rod to the piston to set the rubber seal in the correct position just behind the compensator port. With the factory settings on both of my original boosters rod and the NOS master with the correct factory pistons, it all lined up just like the Master Technician Bulletin mopar used in the training classes. That complete article is on the ebodies.org site. It’s very informative about 70 model brakes. My problems were that the aftermarket master with the shorter pistons put the ports in the wrong position for air or excess fluid to get out of the secondary piston bore. And yes, the bores for front and rear are both the same.

The only mention of pressure in the service manual is in checking the proportion valve and references 500 lbs but never gives a max amount of pressure. I’m not sure where to go next on this but testing in different locations is possible. It almost feels like the rubber lines are expanding but I’ve held on to the lines when my son presses the brakes and I can feel no expansion. I don’t see the calipers moving, either. If it was bypassing in the master, maybe but I don’t see that. What confuses me it the fact my Cuda stops like you hit a wall with only 650 lbs of pressure. It locks the tires up. Of coarse the Charger does stop straight but it feels like anti lock brakes. I’m not giving up. I’m still hunting. Rebuilding the valves helped some.

One more thing. I can’t adjust the rod from the booster like people say on the internet. There is not enough adjustment to move it back far enough to “ just have it barely touch” or have “ .020 of depression. They did not put enough threads on the adjustment. The only way I could do that is to shim the master away from the booster and at that position, it leaves part of the rear most seal visible and that can’t be good. The factory booster and master have a seal to prevent moisture from getting into that place. And again there are no instructions from mopar service manuals or tech training on how to adjust this situation. My biggest mystery is how my Cuda stops with 650lbs with factory parts and this Charger can’t with 800lbs and mostly the same parts but all factory parts. Remember the NOS master did the same thing. I may go back and rebuild it with a new kit and seals to see if that helps but next I think I need to look at the calipers, pads and rubber lines.