I rebuilt my rear wheel cylinders last year.
What a PITA.

I made the special Mitsu Cone Tool to slide the o-ring onto the pistons, you Really need it ,too.

I cut a 3 in. tip off a plastic funnel to slide the rubber over , thereby streching it over the end of the piston. It's probably the only way to do it without damaging the rubber.
In retrospect(I did not know this at the time) I should has just gotten a Gen 2 rear with the easy to maintain disc brakes.
I inspected my shoes recently and since I couldn't strike an arc on the shoes only about 3 in. on top and bottom of shoe were even making contact with the drum. Drum brakes are soo dinosaur.