How can I get dried Elmer's white glue out of hard bristle paint brushes?
Soaking them in warm/hot water is the common procedure for removing Elmer's glue.  Elmer's glue is specifically water-soluble — so if it isn't coming out then maybe it is not Elmer's.

Make sure the water is warm/hot throughout the soaking. 

You may need to peel-off the outer layers of glue, first, and then re-soak to loosen the next layer.