rear brake locks up on 2003 Ford Windstar

why is my rear drivers side brake locking up. New brakes, made sure no air in line and the abs light is not on. Let up off peddle they release ok but when u put just a little pressure on it the brake locks up while driveing on gravel or snow dont notice it driveing on paved roads.

Asked by for the 2003 Ford Windstar
could be adjusted too tight, emergency brake cable may be seized and not allowing the shoes to seat correctly or the drum is out of round and needs to be resurfaced.

I dont think its the emergency brake. I checked it and it seems to be working fine. Could it really be the drum? It is old and rusty replaced every thing but them.
i have the same problem . what is it ?/ can't figure it out