brakes quit working periodically on 1990 Ford F-150

I have a 78 f150 and all of a sudden last night the breaks quit working when i got to the store.nothing at all they just went to the floor. thank goodness i was going slow.... anyway i checked everything and fluids a good no leaks. so i left store easy and brakes worked fine... then when i got home i went to stop and they quit working again.. i have no idea where to start on this,

by in Roseburg, OR on June 15, 2012
1 answer
ANSWER by on June 15, 2012
POSS MASTER CYL IS LETTING BRAKE FLUID BY PASS. TOW TO YOUR MECH AND HAVE IT CK'D