Danny;

I'd recommend hydraulics over air pressure also...you can build up and store a serious amount of force with air because of the compressability...so extreme care is called for! Fluid on the other hand, is safer in that when you release the lever (on for instance, a master cylinder chucked into a vice, and plumbed to the caliper), there is no stored energy to send things flying uncontrollably...

Also, the first piston to pop out will open the system and prevent making pressure with which to persuade the others to move...so you need to control the pistons when they start to move...I like to work with strategically sized wooded blocks placed between the pistons...

Once pistons are removed clean and inspect everything...then you'll have a better idea of what you'll want to replace...

Reinstalling with lots of lube and care is straightforward...

Cheers