Weird problem

I have a 1970 Duster with front 4-piston disk brakes and rear 10" drums, and since I got the car the brake pedal would move about 3" with little resistance, then hit a hard spot and the brakes would start to engage. I noticed only the fronts would lock up, so just over this weekend I replaced a wheel cylinder in the back and bled both sides in the back, and the pedal is a bit firmer now. BUT, it only firms up nicely if I push the pedal down, let up, then quickly push down on it again. When I do that the pedal gets noticeably firmer and doesn't go as far, and it feels like my back brakes are engaging. Does anybody know what could be going on? After they firm up and I let off for a couple seconds, it gets soft again and I have to repeat the process. I'm thinking that it's the master cylinder, because that's the only part that's left that I haven't looked at (it's also the incorrect one, it has the "clamp" that comes over the top to hold the cap on instead of the bolt through the center).