I'll jump in here too..
I don't know if it is a design problem or a quality problem, but the pair of NAPA cylinders I bought was the solution for me. I have about 600 miles on my car now and the brakes are still working perfectly, with no additional bleeding! The only problem I have is that the switch is still stuck. I have been hoping that it would "magically" pop free, but it doesn't look like that is going to happen.
My car definitely does not have a residual valve, and I am convinced that it never did. I still have the original distribution block and there is no valve in it (the car came with factory disc brakes.) So the design really relies on the cylinder seals to prevent air from getting sucked back into the system.
I know its a real PITA to replace them, but I suggest that you try a pair of the NAPA-style wheel cylinders before doing anything else. Chances are that it will fix your problem.
Walt