2002 2500 Random loss of Brakes
If it drops to the floor the master cylinder is internally leaking. If the engine is off you will still have brakes. It's harder to press because the booster isn't working without the vacuum assist. The other reason is you have lots of air in the brake line and/or the rubber hoses are stretching. Was the MC bled when new?
I bench bled the new MC before installing, I also replaced the booster because I in the past have not done that on a failed MC and then 6 months later the booster starts leaking and I have to do it all again.



