From your description it sounds like when the gear is fully up, the cross bar on the end of the cylinder rod is jamming in the cutouts in the side frames at the end of stroke. The fact that the gear releases more easily when the pressure is low points to this as there isn’t so much force to push the cross bar into the jamming position.

There are a couple of reasons why this might be happening. One is that the cross bar reaches the end of one slot before it reached the end of the slot on the opposite side. Then what happens is the cylinder forces the cross bar until the end of the other slot is reached twisting the cross bar and jamming it. The way to check for this is to manually push the cross bar to the fully up position with no air in the system and check that the cross bar reaches exactly to the ends of both slots. If it doesn’t, it might be that the side frames and the cross pieces that join them are out of alignment. It’s worth loosening all the screws holding the side frames and seeing if you can realign them before tightening the screws. If this doesn’t fix the problem your next step is to find a round file that fits in the slot and carefully file the shorter slot until it matches the longer one until the cross bar reaches the ends of both slots exactly.

If you’ve checked and the cross bar is reaching the ends of both slots, then the problem is the shape or surface finish inside the ends of the slots. The fix here is to use a piece of say 400 grit wet or dry sandpaper to smooth the insides of the slots and very carefully ease the point where the cross bar goes into the lock position so that it can slide out when down is selected. A good application of grease can help.

Unfortunately a lot of the engineering in the model world leaves a lot to be desired and leads to this type of problem!

Hope this helps,

Malcolm