Hole in front brake piston...

[R-man]

The 'plastic' disc is ceramic, and fragile. They are designed to limit heat transfer from the pads to the piston and hence the fluid.

It pays to never prise the piston away from the disc without first taking the pads out (or even better remove calipers form wheel), then you have just enough room to press the pistons back by hand...and if they won't move, its time for a really good clean up.