1999 Suburban - Brakes

[GaOkie]

We have a 1999 Suburban original owner 170K+ miles.
To date this vehicle has been Great. Water pump, alternator, fuel pump that’s IT! Knock on wood. Although I do need to work on the door locks and passenger window. Those will be my spring project.
Brake issue(s):
Let me start with break master cylinder fluid level is full. No apparent leaks front or rear.
1. When coming to a stop everything feels fine. Some times the brakes feel a little soft but work fine.
2. While seating at stop light with foot on brake, vehicle can start moving forward with out the pedal going to the floor. You can press harder on the brake or pump the brake back up to again stop vehicle.
3. We have taken the cover off the master cylinder and found the rubber seal no longer located where it should be. Relocated the gasket and replaced the cover. Thought this was the problem at first but not sure although it keeps happening.
Ready to change master, booster or what ever is needed just looking for direction.
Your assistance is appreciated.

[MT-2500]

If brake pedal seeps down at stop light or sitting still with engine running with light pressure on it.
Replace master cylinder.