Power drum brakes questions??

Put a vacuum gage on the intake. If not enough at idle, the power booster won't work. That is common problem for people that install a wild cam. I forgot what reading is needed but you can find in posts. If ~half pure vaccum on the gage, should work the booster fine (15 inHg?).

Unlikely you have a diagphragm leak since you can usually hear as a hissing near the brake pedal. Also, they usually get into a mode when the power brakes grab suddenly as you press it, when a crack in the rubber seals.

Next look at the fluid side. See if you can bleed from each wheel cylinder. You just need a little to prove the lines aren't clogged.

If not above, you are into the mode of replacing parts. I would start w/ the vacuum booster since the most likely culprit and not super hard to replace, especially since yours is probably on stand-off brackets like most Darts.