Ran into same problem with manual SSBC front brakes before. A customer came in who had installed a set, it took both feet to stop this car! We tried various parts, different sized masters, new brake lines and hoses, disassembled prop valve, cleaned, retested still didn't work, tried everything we could find or test. Went through all Feets stuff too. Bought brake pressure test gauges, all good, Ssbc told us there was nothing wrong with their parts. Customer finally gave up, put Baer parts on, car stops like a vette.