[JMFL36]

Hi all. I'm looking for some advice on tracking down an issue with my '54 Mercury power brakes. I am experiencing 2 symptoms while braking.

1. When braking, the car slows normally till the pedal is depressed about halfway down, then brakes sharply as if I've slammed on the brakes.
2. After braking, the brake pedal does not return as it should, but the brakes do release. When I manually raise the pedal with my foot, it feels like there is some vacuum or hydraulic backpressure to my raising the pedal.

I'd like to narrow down the possibilities before tearing into the master cylinder. It looks like it was rebuilt not too long ago (fresh paint and gaskets). Attached are a couple of drawing from the shop manual. Any advice would be greatly appreciated.