I chased brake problems for a solid year on my blue Hornet. When I went through the brakes the first time, everything was dry, so I figured I'd save a buck and not do wheel cylinders. Bad move. That bit me in the ass. A year later and a leaky cylinder was contaminating my right front causing exactly what you describe.

I also had a two-fold issue where the previous owner had put on a new master cylinder. Assuming they'd done it right, I never bothered to check but boy did the pedal feel light until right near the bottom and then it was brakes FULL ON.

Turned out, they didn't know enough to remove the brake rod retainer from the master cylinder. The car had power brakes, but either they got a manual brake master or all masters now come with that rod retainer. Regardless, you're supposed to remove that retainer if using on a power brake car.

Otherwise, if you don't, the rod in your power booster gets jammed into the back of the master and won't return to where it's supposed to.

So, it may not be a bad idea to double check that and while you have the master separated from the booster, if it's adjustable, be sure you don't have any pre-load on the master cylinder.

The master should be able to slip onto the booster without any force. If you have to push on it, or cinch it down with the bolts, the rod is too long.

Good luck!