Delayed Disc Brake Release

Chuck Henderson said
also, do any A-Bodies have brake pedal return springs other than the spring(s) in the MC?

Not that I'm aware of.

When you replaced the master cyl. Did you get a disk / drum or a drum / drum

If it's a drum / drum there is a residual pressure valve in the output port that you are using for the disk brakes.

That could cause the front disks to stay energized for some time.