I have a '70 Chevelle and converted from manual 4 wheel drum to power front disc/rear drum brakes. The instructions (which are for 64-72 gm cars) state to install the pushrod into the lower hole but a different sheet included with the directions say for '70 and up to install in the upper hole. I installed in the lower hole (didn't read the 2nd sheet until tonight) and the back brakes lock up under medium to hard stops. I bought a hydraulic gauge and I have 700 psi at the front and 750 psi at the back. I might as well try the upper hole but want but want to make sure this won't damage the master cylinder. Any help is appreciated.

I would call whom ever you bought the kit from and verify.... Stock power goes into the lower hole. AS far as the brakes locking up, sounds like a proportion value issue.

The correct hole would be the one that puts the push rod center line in direct alignment with the center line of the master cylinder piston.