Power brake conversion issue

Did you remove the brake lamp switch first? If not, the pedal may be bottoming out on it so the MC piston doesn't fully retract. If so, the fill ports may not get exposed. If correct, the tubes should be open to the reservoir with the pedal off, thus no pressure build-up possible. If not, you will not only lock-up pressure in the tubes, they will not refill so if even a tiny leak the pedal will keep dropping until you have no more brakes. Amazing how many people here don't understand the brake lamp switch and how to adjust it. It should not affect the pedal, only sense it.

Most Mopar factory power boosters have an adjustable output tip, as I recall. I would expect an after-market would be even more likely to have one. Unbolt the MC from the booster and look again. Same deal if it is too long, you will stop the MC piston from fully retracting. Adjust until you have just a slight amount of play, just like adjusting the brake cable on a road bike.