Hello :)
happy new year...
After I found my problem (water in the car) in this forum I take a look for other topics and found this one...
I guess the problem is not the caliper piston, the problem is the sliding bolt on the downside of the caliper...
My 450h also have the same caliper problem. Before I buy the dealer tell me that they replace it 20000km before because the calipers are stuck. So I think its ok.
After 6months "in my hands" I do the maintenance (i prefer to do the normal maintenance by myself) and recognize that the rear left caliper (only the left side) dont move.
So I fix it and thought it will be done...I was wrong!
6 Month later I change the wheels, take a look at the left side and: stuck again *hhrrrmgpfff*
Fix it again, add some breake grease there to prevent water comes there...
6 Months later...doesnt help...
Now I fix it again, use more abrasive papersheets to clean the sliding bolt, use a special tough grease and use too much of this grease(until it squeeze out of the rubberseal of the sliding bolt) so water cant be there and stuck the caliper again...
Now, 1year later I can tell: SUCCESS! The last work helps until now. This year I change the rubber seal because I think too much of grease isnt good for that-but its way better than stuck calipers again ;)