For what it's worth -
My understanding is that most are not truly 'rebuilt' but rather just recertified. The dry pumps have a carbon graphite based material used for the pump vanes. The graphite is designed to be self lubricating, and is in fact designed to wear away gradually over time.... predictable enough wear rate that they know roughly when they should be about worn out...
Most do not recommend or even support rebuilding the pumps.
Here is a great article on pumps - out of copyright consideration - I'll only paste the 'link'
http://www.avweb.com/news/maint/182905-1.html