1999 GMC Yukon pedal stops car but slowly goes to floor

I took the car to my local service garage that has serviced my cars for 20 years. He says that it sounds like the master cylinder, so i leave the car for repair. When I go to pick the car up he says that he tried three new master cylinders, but the problem still exists. He says that it must be the ABS system and that i must take it to the dealer. I stopped by to talk to the dealer service mgr and he says that it does not sound like the ABS is the problem and that the car should still have the brakes even if the ABS has malfunctioned. I agreed that i should take the car in for a checkup. If it's not the ABS causing the problem and there's a new master cylinder installed, what else could the problem be?

Thanks