I have this same issue with my 95 cc350. Brakes work great similar to RubberDuck. But when sitting still with engine running if you apply extra force to the brake pedal, it will slowly go to the floorboard. Something is wrong!! I have checked break pads and shoes, checked for leaks and all looks good. Vacuum pump was replaced due to failure yr ago. When this happened it was VERY difficult to stop the truck. My thoughts are if the pedal is going to the floor fluid is going someplace, right...??? If there are no leaks then it is by passing someplace. Can the abs valve do this? Or is the master cylinder by passing? How could a bad booster cause the pedal to go to the floor? Looks like the line pressure would go through the roof causing a wheel cylinder or caliper to blow a seal.

Anyway, I would be very curious to what is going on. I have read several other posts with the same topic but have not seen a good logical fix.