73 Plymouth Scamp brake issue

[Hawkfan#12]

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

 

[Steven190]

Check the vacuum that the engine or the vacuum at the booster

 

[Demonracer]

What is the engine build, cam, intake, etc. It sounds like a vacuum problem but more information is needed, & pictures of the vacuum line to the carb or intake.

 

[cudamark]

Sounds like the valving in the booster has gone haywire to me. I would start by checking the push rod length with your new master cylinder first though. Maybe wrong master?

 

[65TerrorCuda]

Buddy of mine had that happen after the dealer serviced the rear drum wrong on his Yamaha. Doing 55 he touched the brake and it sucked down the pedal and locked until he came to a stop.