ORIGINAL: JeffH

Its a matter of careful construction with regards to installation techniques...ie keeping rails parallel and square to eliminate any binding in the frames.
Second, you need to use quality retracts, hoses, tee fittings, and valves, both retraction and fill. Cheap hoses can split and kink, cheap valves leak.
Third, make sure you have enough volume to adequately cycle your gear. I prefer 4 cycles per air charge. Your valve must be able to pass enough air to raise large heavy wheels and brakes. IMO, the Robart valves are not up the task. I would go with a BVM or Sierra high volume valves.
Finally, maintenance. You have to lube the cylinders once in awhile, yearly??? You must keep a check on how flexible your lines are. I have seen planes go for years on the same quality hose, BUT these planes had gear systems that were never disconnected and the hoses did not see UV rays. One of these was a 101" B-25. The wing center section was permanent, so the air system was never cracked open, and the hoses were never exposed to UV rays, so they lasted for the life of the plane. I dont think that plane EVER had a retract failure in the 3 years or so that it was flown. Also, dont be afraid to disassemble cylinders to replace O-rings. Things wear out, or normally, they age out with O-rings. They normally get hard from sitting, or lack of lube, or just plain age. Robart sells rebuild kits for their cylinders, its easy and cheap to do.