if you are using a tool such as a caliper compressor or a large set of channel locks, they will easily compress with the brake flujid cap off. be prepared, a little brake fluid will drip from the resevoir as this happens. if they do not, they are bad.
if you are working on the rear calipers, they need to spin in, like you were screwing the piston back in instead of compressing.
Technician
schooled, trained, worked in industry for years. specialize in the odd issues