I installed a new set of Robart pneumatic retracts on my P-51 and after a few flights, one of the retracts is sticking. That is, rather than fold up and down easily, it requires quite a bit of pressure--enough pressure that it can no longer be operated by air.

I've taken the retract apart. It doesn't appear that the mounting brackets are interfering with the retract mechanism, and as far as I can tell, the shaft going into the air cylinder isn't bent, either. Has anyone experienced anything like this before? Is there some kind of lubricant I should be putting in the air cylinder or anything?