I have been having some problems with the brake pedal not popping back up after driving for a while. Then the brakes would start going almost all the way down to the floor before stopping. I bleed them and it got better and figured there was still some air in there and would try it again. I bleed them for the 4th time to try and remedy the problem and now I can hardly press the brake pedal down. I used a pressure system today (for the first time) and the rear went smooth and the fronts did not. The front flowed and did not have any air in it, then I went to the drivers side and it was the same. I thought that I better go back around one more time and just make sure. Went back to the front drivers and could not get anything to come out. Tried the drivers, same thing. Pumped the brake pedal a few times and tried a self bleeder and had fluid coming out, driver and passenger. After I was done, went to back out the driveway and had to press the brake pedal really hard and it seemed like I was not going to stop. Figured maybe I could cruise around the block and it would get better, yea right! I was wondering if it could be the prop valve? I am puzzled, any ideas? Thanks!