First I’d like to give George ( dragon slayer) here on the board for pointing me to all this information.

I’m going to discuss this setting the push rod correctly ordeal. The service manual gives no information about this from 68-70 on b body disc brakes. The only reference is for the different Midland Ross drum brake application. That master cly has a stop installed so the piston is flush with the back of the master so it has has a specific adjustment. That’s not a topic here. I’ll post a picture of three different masters. The far left is an NOS 70 ebody disc brake that has the stop. You adjust the booster pushrod with slight clearance so you get the full stroke. The piston cups are held in the proper with the stop. The other two are what we are discussing here. The middle one is my NOS 70 bbody engine side exit master. The far right is the reproduction with engine side lines. Both of those have the piston sticking out from the base .147”. I have two oem unrestored 70 Bendix boosters and they both have the rod out .925 from the base of the booster. So, when you install the master to the booster, it pushes the primary piston of the master to a FLUSH position. The reproduction is slightly different by a few thousands because of the depth of the hole in the primary piston.

The reason the adjustment is critical is what I’ll explain. I’ll post some pictures of the workings of a master cly from a Mopar master technician instruction manual. It shows that the piston cups, when installed in the correct position, are directly behind the small hole in the MC called a compensator port. It allows excess pressure to bleed off into the reservoir and lets excess air to bleed off as well, IN THE BRAKES OFF POSITION. Also, the brakes will not build ANY pressure until the cups PASS the compensator port. In other words, if you adjust these two b body MC with clearance between the rod and piston in the off the car position, it is wrong. Until the cups pass the compensator port, all you are doing is push fluid right back into the master reservoir.

Also notice you can slightly see the rear seal on the b body MC in the off position. That can’t be good. Also notice in the diagram that the rear piston is FLUSH with the back of the master cly.

This post not be of relevance, because this MC is not available anymore, but I’ll show it anyway. The MC I’m referring to is the A1 Cardone part number 13-1475 for a 70 b body with engine side outlets. It has the wrong pistons installed as I received them. The secondary piston cup installed in the static and factory pushrod location has the cup PAST the compensator port. So it cannot bleed off excess pressure or let fluid back in. The casting is good and machined well. It’s just got the wrong piston in the secondary position.

Now this will work but just not as intended. How much it affects it, I do not know. There is a fix to get it right. So, here we have the pistons removed from two MC. The top gold one is from my NOS one. The middle is from a Raybestos kit no MK 620 for this same MC. The bottom one is from the 13-1475. Notice how much longer the piston is on the bottom one. Installed, it goes PAST the compensator port. You can visually see this( with good eyes) but I checked it with 15 psi of air press I the port and got no air out. Now there are two ways to fix this. One, buy the Raybestos kit and change the piston or two, adjust the rod in the booster SHORTER by .040( in my case) . That will uncover the port in the sec but will also move back the primary. It will make the piston travel to the to block the port to the front brakes by the same amount.

However, all of this discovery has not solved my brake problems. I’ve installed my NOS MC and bleed it several times. I’m now looking at possible problems with my proportioning and metering valves.

Very interested in your results on this, please keep posting. I have a similar problem with my 69 Charger. I have literally swapped every brake system component over the years, had the booster rebuilt, etc and made only a marginal difference. I was able to improve the situation by using a brake pushrod adjusting tool, a power brake bleeder tool and a switch to a 1/16" smaller bore master cylinder which gave me a stronger brake feel. My problem is made worse by a cam that gives me poor vacuum.

Couple of questions, how many miles have the vehicles been driven since brake work? Mainly are the pads and shoes bedded in?

Also, how does the arc of the shoes match the drums?

I know in both cases these usually present as softer pedal poor stopping until everything is mated up. But, it's worth checking.

Note, the alignment of the secondary piston can be "adjusted " by shortening the spring between the pistons or a shim if needed. This can probably be dialed in by dropping a piece of wire through the compensation ports as a stop and a dial indicator to measure endplay.

I can't believe we need a thread on blueprinting a master cylinder.

Good information. In car repair I've always said if you don't know how something functions It's difficult to repair. So many people out there that do repair ( I'm talking actual techs as well) with very little knowledge of how a component functions. That canmake diagnosis hit or miss. Been doing this stuff 50 years and I'm still learning. As a side note; I have seen the factory proportion valve or the pressure light activation switch assy sieze internally.
Doug

If I adjust the primary spring shorter, it might move the cup for the primary past the compensator port. It would shorten my stop inside the primary and give me a little less front brake stoke. Shims under the secondary spring will not overpower the primary spring either. I think the only way to solve the problem is to shorten the pushrod from the booster. Yes, it’s hard to believe we are even discussing this remember we are talking about reproduction parts here. The nos mc works with the factory booster. Neither one of these are even available anymore. Just used or rebuilt factory parts.

I may have made some progress today. The last time I bled the brakes on the NOS mc, I got all fluid. I had my bud press the brake and the rod on the metering valve did not move like it should. I drove the car home today and did some hard stops. When I got out, I could smell the brakes. The front rotors were not hot but the rear brakes drums were so hot I could not touch them. I think my metering valve and proportion valves are both bad and stuck. I think the back brakes are doing most of the stopping.

As a matter of fact, I looked at the rear shoes to check for wear and to adjust them again. They were not really making a great contact pattern but I don’t know if anyone still grinds them in the correct arc anymore. They also had some vertical cracks in them. Probably from excess heat. I need to get new ones.

Sounds like you have multiple problems. Is this car a restoration or a driver?

If a driver, I'd simplify the braking system. Run the front brakes straight to the master and an adjustable p valve for the rear.

You can easily check the arc match up by holding the shoes in the drums, you should have close to full contact. I agree that finding someone to match the arcs can be troublesome but clutch places might be an option.