The air pop did not work on a pair of rusty calipers from a Spitfire so I used this Piston Popper KD-2105
(like one here https://www.ebay.com/itm/K-D-Tools-pn-2105-Piston-Popper-Disc-Brake-Stuck-Caliper-Piston-Retractor-USA/303544941278?epid=732619622&hash=item46acb03ade:g:YEAAAOSwV7ZenImQ )
Worked great - unique tool but was the only thing I could use to get the pistons out - no lost fingers, no spray of fluid and only minor swearing