My 1994 isuzu trooper brake pedal goes to the floor. i replaced power booster master cylener bleed the brakes they still went to the floor. then after 2 days went back out and bleed them again and they started working.worked for about a week then they started going back to the floor again. no leaks anywhere.checked the vacuum line check valve seemed o k. no signs of cracks.bleed the breaks once more.lots of pressure in dack celipers but not much pressure in the front calipers help. disk brakes allfour