Delayed Disc Brake Release

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.

Unless it's a really bad residual pressure valve, it should hold pressure for a lot longer than the 3-5 seconds that the OP describes...