I have a '73 Datsun 240Z and am having some trouble with the brakes. Without the vacuum assist the brakes are VERY heavy but work fine, about like brakes should work without vacuum assist I guess.... The problem comes with the assist hooked up. The pedal is as light as a feather, it travels half way to the floor without doing anything, and then in about a 1/2 inch travel the brakes nearly lock up! Very annoying. I put a new master cylinder on have bled all of the brakes with no improvements in my problem. Any ideas out there?