On-ground Pressurization Test

[Stratus337]

I'm trying to trouble shoot a bit more before starting the work to hopefully point the mechanic in the right direction. I flew yesterday on a 200 mile trip each direction (out and back). Because of winds I went down at 11,500 and came back at 6,500.

At 11,500 I was getting a 1.5x dif with a cabin around 8,000.

At 6,500 I was getting a 1.0x dif with a cabin around 4,000.

If I had a true leak I would have expected the dif to get to the same maximum level. With the lower dif at the lower altitude it makes me think that something may not be communicating correctly between the outflow valve and the controller. I don't have the service manual, but are those the only two components that ultimately control the outflow valve and the differential pressure?

To me, it appears the outflow valve is not working correctly to maintain the inputted pressure commands.

Any help would be appreciated if the facts above point to something someone else has dealt with.