It's one of your brake caliper pistons seizing
you should be able to work it free
if not its a £20 note to uncle melvin and a seal kit to fit
how do I know......
guess what I'm doing to my car tomorrow morning
I fitted new pads and when I backed the pistons back into the caliper the Fos inner was stiff.
car now pulling to left under braking
Time to strip / clean pistons and rebuild
Messy job
