Waltec diverter issue

[ggg1379]

Have a question...bought a new house last year(new to us but 100 year old house) and recently the diverter had gotten harder and harder to push in to the point that my girlfriend is having shoulder issues from having to put so much effort into it. I finally got around to looking at it and when I took the diverter out of the wall it looks almost brand new including the o-rings so I just tested the fluidity of the mechanism and it felt much smoother than it had installed. So when I re-installed it, for a few tries it seemed to be functioning normally and no longer a pain to push in when suddenly it kind of popped and was no longer diverting water up when in shower position. When I pulled it back out the o-ring had come off the end of the piston and was stuck inside the cartridge. I managed to get it out of the cartridge back onto the end the piston, re-installed it and it seems to be working properly but for some reason I am not convinced it won't happen again. Any thoughts on what I might be doing wrong? Should I try replacing the entire cartridge or did I do something wrong when I re-installed it the first time?