herman miller is very good about sending parts. you have to pay of course. the piston is not a real expensive part. it is just a friction fit. i don't see why it would need to be welded. maybe it is something else like the "control" is cracked. that is expensive.
oh wait! i looked at it. i am taking a guess. the "c" clip may simply have broke and come off! if that is the case you can fix it for less than $3! that is really what it looks like!
so go over there and see if the piston goes into the hole in the base. the hole in the base is slightly bigger than the rod on the piston, it is not a tight fit. if that is the case and you get it for say $250 you stold it! good used ones are like $700.