i have a 1974 chevy C10 stepside. today I had to do a quick stop and the rears locked up. Seems I had no brakes on the front. This seems to happen only in a quick or emergency stop mode, just when you need all the braking the most. Is this possibly a proportioning valve issue. Brakes were recently overhauled to include rotors, drums, calipers, brake cylinders, master cylinder, and vacuum assist for power brakes. Everything was good 3000 miles ago. I suspect the proportioning valve as it seems to be the only thing not new on the brakes. Also...any tips to changing it out would be appreciated. I suspect just disconnect the lines and wire and install the new valve and then bleed the system?? Thanks for any input..