Raf, I have the same UP3 valve in my F-16. Granted, it has only 3 gear doors but it always works perfectly. I know my system in the Chippy Ho isn't leak free since it looses all air in about 1h. So maybe a leak from a cylinder could cause this. I agree that a dreamworks electronic valve would be the way to go but it would be a big change.

I'll try to troubleshoot some more and will report back. My plan is to pinch lines to isolate what cylinder might be causing this. I also have some BVM "magic lube" that I can try to use on the door cylinders and the gear.

Jesus, the valve has 2 outlets for each output (gear down, gear up, door open, door close). I have placed a T fitting to join the 2 outlets for each output. YellowAircraft is suggesting to remove all Ts and use one outlet for a subset of cylinders and the other one for another subset.  I might try that but that would be a lot of work and it is hard for me to understand why it would make a difference.

Thanks again for all the help and please keep the suggestions coming.

Arnaud