You can try this, put a 15/16 socket into the piston and knock it in using a wooden chock with mallet, then put socket handle in to the socket and work it in and out as you would when undoing/doing a nut. Use wd40 liberally. The piston will come out in 5 minutes. The downside is you can't get the socket out of the Piston! Well I've not yet tried, but there's sure to be a way.
In the end better to lose a socket and piston in the short term than an entire caliper.
It is a 6 monthly check because of the pistons exposure to rain etc.