How can I fix my JCB 520-2 Loadall?

JCB 520-2 Loadall Year 1982.

Just recently the drivers side front brake has seized. Not sure where the problem lies. The small piston/mechanism behind the foot brake appears to be working so suspect that problem is at the other end. When I open up the bleed valve at the wheel end and let some fluid out this releases the wheel, but when I tighten it up and apply the pedal, the wheel seizes up again. The brake mechanism lies behind a circular plate held on by about 8 12mm bolts. What do you suspect the problem might be? and would it be ok for me to take the plate off to make a further investigation. My knowledge of car disk brakes makes me think that a cylinder or piston of some sort has corroded and is jamming.
Thanks for your time and any light that you can shed on the situation.
Regards
Paul Inman