Greetings Don:
Probably the easiest and best way to handle this issue is not to do anything at all. Most procedures used to fill and/or drain these water-holding cavaties are cosmetic and often lead to more/future complications.
However, sometimes this answer doesn't sit well with some homeowners -- in that case, it is generally recommended to contact an arborist who is certified by the International Society of Arboriculture:
http://www.treesaregood.org/findtreeservices/FindTreeCareService.aspx
Best of luck!
