I just bolted on a new master on my 68 with front discs. I took it for a drive and it seems to brake fine but when i let off i can fell it continue to brake for a second or two, then i feel them let go.

What could this be? any ideas?

Does the pedal stay down, or does that come back with pressure against your foot?

I had a bad power booster, so when I'd hit the brakes the pedal would stay down for a while.

A bad brake line to the front wheels will do that too..could be collapsed on the inside won't let the fluid go back to the master cylinder.