To find out whether the roll control is leaking or the master cylinder, first the brake pedal needs to be depressed at about half pressure with the roll control button not engaged to see if the pedal creeps to the floor. When pedal is fully depressed, that can cause the rubber piston in the master cylinder to expand gripping the bore more tightly. With about half pressure applied that doesn't happen, making it easier to determine whether the cylinder needs to be rebuilt or replaced. If the master cylinder checks out to be ok, the roll control becomes suspect. Quite likely, it didn't get flushed thoroughly before instillation, or if it did, the internal parts may need replacing. I have seen repair kits listed, so it must be a common problem.