Master Cylinder Leaking

I tried the search but didn't see something specific to my problem.

The car is a 1972 Swinger, it has 74 Sport front discs, and I recently swapped an 8.75 in. I took the backing plates off my 7.25 to reuse the brakes, but when I got everything back together, the car struggles to stop and the master cylinder leaks out of the cap and melted the paint on my inner fender. It didn't do this before, so I figured maybe I mixed the drums up when I put it back together. I adjusted the rears and it stops better but still takes a lot of effort and leaks. They're bled and all. What gives? The only thing I can think that's different is my parking brake isn't as tight and I need to snug it up, but I don't see that causing a difference.

Any help? Thanks!