I am sorry to hear about the issues you are having with your steering system. I went through the same issues and found it to be extremely frustrating. I can't speak for everyone but can tell you how I solved the problem on my C 28.
After replacing two pumps I decided to install the check valve from Garmin. This was almost 100 hours ago and it is still working well. The check valve isolates the autopilot pump from the manual steering system through the use of 2 robust one way valves. These valves allow for the flow of hydraulic fluid from the autopilot pump, when activated, but does not allow for the back flow from the manual system back through the pump.
I installed my check valve with "tee" fittings on the manual side, to allow direct flow through the lines, bypassing the pump manifold, and used 2 one foot hydraulic extension hoses to connect the autopilot pump to the opposite side of the check valve back to the pump. I have pictures of my installation in my gallery but it is difficult to make out the details so I've added a better picture from an installation I found online. I used this same picture to help me with my install.
I hope this helps solve your steering issues. I know it can be frustrating but it seems, so far, that there are ways to fix the problem.
Terry
