I have a 70 roadrunner with factory disc brakes ..The booster went out last week , so i replaced the booster and master cyl with remaned units from Advance Auto parts ..The pedal is as hard as a rock and the car will not stop .. The paper work with the booster said to adjust the rod longer or shorter as needed to work correctly .. I have adjusted the rod from all the way out to all the way in half a turn at a time .. I have had the master cyl off at least 50 times trying to get the rod set right .. NO luck at all !! Making the rod longer makes the brakes hard as a rock and will not stop .. Making the rod shorter the brakes will start to work and stop ok BUT the brakes drag .. The more you pump the brakes it the worse the drag gets . I have never had this problem before with mopar brakes .. Anyone help ? Thanks Lee..

At the risk of oversimplification... could there be insufficient vacuum? What about trying the old master cylinder as a test? Once again, not trying to insult your skillset, but did you "bench bleed" the master cylinder first?
If the pushrod is at the shorter setting, I would guess that it lessens the travel, and therefore the master only actvates the front circuit?
I had manual 10" drums on my 70 Charger when I got it. I converted the front to power discs from a 75 Dart... Booster, brackets and everything. Now that I have the MP 509 cam and its piss poor vacuum, I am considering a manual disc setup. Sure, if your car is original I could see wanting to retain the booster, but you do have options if the power stuff cant be tuned to work properly.

Thanks .. Yes bench bleed as well as on the car.. Vacuum is 17 at idle ..Rememeber it had power brakes that worked fine till the old booster went out .. I Returned the master and the booster today and ordered from Master Power Brakes a NEW booster and master Cyl from them . This is what I get for buying remanned brake parts from Mexico ( Advance auto parts ) ..I was trying to save a few bucks..

People like to dump on foreign made/remanufactured parts. Sometimes they deserve the bad rap, but sometimes they are okay. I sure wouldn't expect OEM long term durability for $25 !