rear brakes

I've noticed for some time that my brakes cause the disco to steer slightly left, had both rear wheels off today to change the shocks and noticed that the offside disc is rusty so obviously not working properly. Any suggestions? I have an EZ bleed kit thinking of checking for air in the system, is this worth looking at or is it likely that the caliper is seized?
I did replace all calipers a few years ago but all were used..
This was not flagged up on todays mot presumably as being a 4wd it cant go on the rollers!
Thanks

some mot stations are set up to allow for 4x4 brake roller test but most will just use the decellerometer to avoid any possibility of damage to the vehicle. it could be a seized calliper that is holding the pads away from the disc and so you will have reduced braking on that corner, so the truck will pull away from that side or if you can push the pads back then it may well be a blockage in the brake line or the calliper. or the pads may be stuck in the sliders so either way you will benifit from servicing the brakes with a good clean up. just be ready for the brake pedal to require less effort to cause the same braking effort. a fluid change can only be a good thing so long as it's new fluid not some thats been sitting in the back of the shed for years as brake fluid sucks moisture through dam near anything and this reduces its boiling point and can cause brake fade.

Cheers mate shall have a butchers today