ON the t's, they frequently have a small bit of slag attached to the inside that can come loose and plug the holes preventing gear retraction. I stopped using the plastic t's because they are hard to stop from leaking. I replaced them all with the brass t. I run a number drill through them to clean out the solder slag and blow them clean with high pressure air. (computer air cans from Wal-mart work good). I lube the cylinders with Marvel air tool oil. The ROBART valves need special care as the nipples are glued and pressed into the valve body along with the adjuster needles. Any stress will crack the glue and the valve can't be repaired. ROBART has been good about warrantee replacement of leaky valves. If you try to adjust them for scale retraction speed they will be too slow with air loads on the gear and may fail to complete the travel and lock. 120 psi is about as high as you can use on hot days, more may cause the lines to pop off. I always top the pressure before each flight. They will go up 6-8 times reliably and come down and lock 12+ times. When adjusting the ROBART valve needles use low strength thread locker on the needle threads and gently tighten the lock nuts to prevent cracking the glue.
I always assemble the entire system and dunk it in a bucket of water to check for leaks. if you remove an air line from a nipple you must cut a small piece off the air line because the plastic tubing will stretch slightly at the nipple, make the installed air lines with a loop of slack for future maintenance.