Drilling a hole in the piston will not ruin the calipers. Pistons will probably have to be replaced if it gets to that point. All the ones that I took out like that, were badly pitted.
Dave
Then tell be what your gonna do if the caliper is stuck all the way down at the bottom of the bore?
Your drill is gonna go into the bottom of the bore, then the same thing with a tap.
People aren't using their noggin when it boils down to just using a bit of high school physics knowledge, and hydraulic pressure.
The grease gun method is "messy" but gets the job done.
And you have to limit the travel as one of the stuck pistons always works it way free first.
"C" clamps are your friend, doing the hydraulic method, or i use a block of wood between the caliper body half and the jaws of a bench vice.
Opening it when when a piston breaks free from the other, keeping them equal in the removal process.