I added disc to me 66 caprice .i used factory 69/70 single piston and still have drum on the rear . Now your problem I would check the vacuum that you have . When I added the power booster I found that there are two holes on the brake arm and the rod from the booster had to be installed in the lower hole . When I added the booster I added a dual marker cylinder and split the system . One more thing a high lift cam will affect the vacuum your engine puts out .some people put on a engine driven vacuum booster Good look .