Are you running a proportioning valve; monte brake booster? What engine is it running. Could be it ain't making enough vacuum for the booster. I had a major prob with a similar brake set up on my 69 Camaro when I put in a 572ci. I had to fit an electric vacuum pump by Stainless Steel Brake Corp. It pumps up the booster and then shuts off. F*&%k did it make a difference.