If I am correct these are a parking brake caliper?
The pistons are threaded internally and have recesses in there face for a tool to grab and twist them in.
I generally use a syringe with ATF to lubricate the piston, you can lift the dust seal to get a bit of lube in there also.
You can make a tool, buy one or use long nose pliers.

But please do not use a C clamp.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.