Anyone have any thoughts on some common problems that would cause the brakes to drag on 5210 series tractors? Could it be valve related or more likely internal issues with discs or pistons/seals?

They have been a common problem… you can try loosing the brake lines and see if they free up… (this would indicate a problem with the brake valve… 99.9% of the time… this is not the problem…)
Normally its the brake retractors… weak/stuck/damaged… causing the braking plate to not retract… There were also issues with the O-rings on the piston rolling or swelling and not allowing them to retract…
If you go in… replace both… or some throw out the retractors…
JD 5000 series tractors have a history of springs(parts key #1) getting weak causing brakes to drag
