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On the LH side that you have off look for the wedge to see if it has been replaced with the old style floaters that look like a link.

If you have the linked floaters they can go over center and then the brakes are locked.

Look through the open areas on the RH side cast iron drum and see if there is a brake wedge or a linked floater.

If you have a linked floater and it is over center ask and maybe someone on the forum may have a solution other than flame cutting the drum off.