Maybe you could put some regular disc brake pads on and see if the braking action improves (after they "bed in"). Otherwise, I'd say it's the master cylinder bore diameter. Don't quote me on this, but I think a bigger bore makes for a harder pedal, and smaller bore makes for easier pedal effort. I used regular NAPA pads and a manual master cylinder from a '70 Belvedere application (no idea what the bore diameter is) and my '64 stops just fine.