Yesterday I had my 5th flight with this P-51 and had two things happen with the retract valve. On one of the instances, the standard servo I was using to activate the valve went bad and started to jitter back and forth. This caused the air pressure in the system to bleed out slowly. The indication of this was that one gear leg would partially deploy when the model was pulling "G" in a tight turn. As soon as I saw this I put the gear down and luckily there was enough pressure left to lock it down and I was able to land w/o any damage. I replaced that servo with a HS-645MG to make sure I had a good servo on that valve- Yeah, I know, its overkill, but I did it just for peace of mind. Then, the servo valve mount glue joint let loose so the valve was just moving back and forth w/o cycling the gear. I took it home and beefed up that mount with some tri-stock. I thought I had a good glue joint there before so I suggest you guys check your valve mount too if you haven't beefed it up a little bit. The valve was not bottoming out on either end of the stroke either, on mine that mount joint was weak and it could have caused a gear up landing. The model flies great, I'm in love with it - it is worth the effort to get'er done!
Have a good one!
Rocky [8D]