So my wife has been complaining of the brakes just going to the floor with no pressure, but everytime I go out to drive it everything works fine. Then yesterday we were driving around and I was the one behind the wheel when it happened. I had to admit it was scary so I pulled over and looked at it when she would press the pedal it would go to the floor and I could see it bubbling in the brake reservoir. About 3/4 of the way home it started working normal again. I have reason to believe it may be the proportioning valve but not sure and can never get it to duplicate the problem. The family member who sold me the car said it happened to him about twice a year and he replaced the master cylinder the calipers the pads and some brake lines the only thing he has not changed is the proportioning valve and the vacuum booster. He is an ASE certified mechanic and works for Ford and he could not find anything wrong with the brake system and said that the proportioning valve is his best guess.