I am having a problem with my GFI circuit breaker. It trips. Now
perhaps it is working correctly, but I'm not sure. The circuit controls
the central vac in the garage (and a couple of outlets in the garage and
exterior of the house.)

I was told by a reliable electrician that one problem might be leakage
due to build up of dust in the vacuum, or in the outlet boxes. I
removed them all and blew the dust out, and took the vacuum apart and
cleaned it well.

The problem may have gotten better, but it is still there.

A new breaker is pretty expensive and may not be the problem. How can I
troubleshoot this? Is there anything that I can plug in to an outlet to
determine if 'this one is the problem'?

Rob