try a c-clamp, but if it seized up. it will likely have to be replaced. If you can retract it with the c-clamp without tremendous effort, it may be OK...test if it works by jacking up the wheel, wheel should spin relatively freely with no brake pressure applied once you used the c-clamp to retract it.... with the wheel jacked up, Have a buddy stick a screwdriver under the trailer tongue to manually pump the master cyl.. brake pads should engage and you won;t be able to turn the wheel..if not- replace caliper.. Then release the pressure with the tab under the trailer tongue to disengage the brakes. wheel should be able to turn. If the caliper is seizing, it will be difficult or impossible to turn the wheel with your hand. This is not foolproof, and the safest thing to do is to replace the caliper.
There are pictures of this in the brake bleeding procedure in the maintenance doc., which you mine as well do if your messing with it anyway.
