I changed out the master cylinder twice, both times with brand new ones, bled a half gallon of brake fluid through each brake line, took off the anti-lock unit and replace it with a proportioning valve and that only leaves one thing. The master cylinder must have, about an eighth inch of preload on it in order to work properly. So in other words, when you put it up to the brake booster to mount it, if the little push rod isn't pushing it away from the booster, then something in there has worn out and now it's not reaching and you need to replace it.
My suggestion is to make your own out of a hardened bolt that is the same diameter as the factory pushrod because I doubt the rod is the only thing with wear and if you buy a new one, you'll have the same problem, so just make one like I did.
You'll have to get a bolt that's about 8 inches or longer so you'll have enough bolt where there's no thread after cutting it. It's best to buy 2 bolts in case you mess up the first one. Cut it slightly longer so you can grind it down little by little until it holds the master cylinder away from the booster about an eighth inch before tightening. Go slow and take your time because when you start getting close, it's real easy to grind off to much and then, you'll have to start all over again with another bolt.