You only need to check the ports that you are using. Having that check valve in the master when you don't need it won't make for hard braking, it will just make the pads drag on the rotors. It is good to check for it. If you find one, there is a way to take them out. Just let me know and I can talk you through that.
The lack of the check valve, and large reservoirs are the charactersitics of a disc/disc master cylinder, so if there are no check valves that master cylinder is fine to use.
It is possible that if the emergency brake is not adjusted properly it could reduce braking performance. I would do a GOOGLE search for those calipers and see what others have to say. The calipers on the 9" Ford rear axles out of the Lincoln Versailles are famous for that problem. Unless the parking brake is adjusted up tight, you will never have good rear brakes.
Keep us posted on your progress.
P.S. That combination valve looks pretty low to the ground. It isn't gonna drag on speed bumps or anything is it?