Im working on a 1970 chev and every once and a while the pedal will sink to the floor while driving.

Ive installed all new parts

Master
calipers
cylenders

WTF is going on? I DID bench bleed the master and obiously bled the system.

Thanks Dan

have you checked your booster vacume (sp?) for a leak? sounds like to me you got a bad booster.....off to the parts store again....:(