If you are on a downward slope it will behave as you described as gravity will keep pressure on the surge master cylinder otherwise should be smooth.
First thing I would do is inspect the brakes themselves as they can get rusty and freeze. Then verify the master cylinder is releasing pressure when sitting on level ground.
FYI when backing up the brakes will lock and there should be a pin to insert on the master cylinder to lockout the brakes.
1997 2400 SCR
5.7 Vortec / Bravo 2