Hi David,
I have attached a sample that illustrates solution for your problem.
CallbackWithMasterPage
When using the CallbackPanel under the MasterPage, replace the __sfCallbackPanel1.callback() to __sfctl00_ContentPlaceHolder1_CallbackPanel1.callback().
In the sample, i have explained the usage of callbackPanel under MasterPage.So, have a look into it and let me know any further queries you have.
Thanks,
G.Pradeep.