Has anyone got enough experience with these to help me identify the problem?

My Sabre is all ready to fly except for a major problem I am having with the retract system. The main gear is Spring Air 503s and the valve is the UP2 sequencing valve. What is happening is that most times when cycled, air vents like crazy from the UP2 valve and the retracts won't even unlock to move until the pressure has collapsed, or else the gear does move and lock but air carries on venting from the valve until the tank is nearly empty.

I spoke to Philip at UP valves, he was most helpful, said it is probably leaking across the piston in the retracts. I did a few tests advised by Philip and by the UP instructions such as pinching the pipes, and it always points to the air leaking across the retracts rather than a problem with the UP valve. There is a test you can do looping the UPs outputs back on themselves, I did that and the valve cycled properly every time. I took the Spring Airs apart and cleaned them in hot soapy water and both were contaminated, one had a tiny stone chip in that had bitten little flakes off the O ring! Disgusting, these were brand new, unused, fresh from the packet. Anyway, I got new O rings, cut new cylinders from brass tube and reassembled them with silicon grease. And the problem is still there, they leak like crazy across the piston. But the daft thing is that if I pump each one up individually, they don't leak, it is only when the pressure is switched from one side to the other that the leak occurs and will not seal until the pressure has gone down to a much lower level.

Although Spring Airs do not need air to push them down, I have to connect the down air line to the valve as it uses the exhaust to control the sequencing speed. So for a test, I disconnected the down line and blocked the valve side to seal it, and left the leg side open to vent. Now on cycling the gear it works as you would expect, the legs slam up and down instantly, but most tellingly there is no leak across the piston! It seems that they only leak when both sides are connected through the UP valve. Could it be that because the UP valve has to restrict the exhaust flow, the pressure on both sides of the O ring is fairly similar so there is no pressure pushing the O ring against the cylinder and the leak can occur, then when enough air has been lost and the tank pressure has dropped far enough, the leak closes. Once the overall tank pressure has fallen quite low, the gear will cycle ok every time with no leaks but by then the pressure is no good to lock the gear up. That would explain why the psitons never leak when I pump them up individually, the air pressure always builds gently on one side and is always higher than the other side, sealing the O ring against cylinder?

Are Spring Airs poor quality? How do I get them to stop leaking catastrophically across the piston?

thanks for any help,

Harry