O rings get dried out and quite often cause leaks. Once an O ring dries out it should be changed because it may have small cracks. One of the most important things to do when doing maintenance is to lube the system. If you fly the plane every weekend 3+ times when out, couple of drops in the lines once a month helps. At season end clean what needs to be cleaned with compress air and lube, keeps O rings and air lines from drying out over the winter. At start of season lube and do a few cycles and check for leaks. Also remember when you get a set of retracts or control valve, you do not know how long it has been sitting and it what climate condition. So assume there is a good chance the lube from when it was assembled has dried out or close to it. So we should do a lube job before we start testing the retracts.