Skidus,

I have a Top Flite P-39 with Robart Air retracts. The front gear door operates as part of the air system. Robart makes a air cylider you can use for the gear doors. they show how to connect the air lines so the door opens before the gear trys to lower and closes after the gear is retracted. It seems to work fine. I hope this is what you are referring to. I don't have a separate valve for operating te gear door/s. I just connected it via the Robart instructions. Check their web page I think you will find it.

Sledge_78