73 Plymouth Scamp brake issue

My Scamp has its original /6 power brake booster, however I switched the /6 to a 360. It has front disc/rear drum and a new master cylinder.

The issue is as I begin to apply the brakes to about 20% the pedal gets sucked down on its own to about 80% rather abruptly. The brakes stick momentarily then will release. Bled, no leaks, level good??????
Any suggestions.

Thank you in advance