Same kinda thing happened to me a while back... After messing around with it for ages and having no joy, I noticed the piston had a really slight groove down one side, I lined it up with the `wield` on the lever body where the two halves are joined and it worked fine. (god knows how it got twisted by it's self?)Prior to that the piston would get stuck really bad, but after lining things up it was smooth as anything!
Sounds like bullshit I know, but it worked!