Do deleted domains lose pagerank?
 

i back ordered a pr4 domain and a pr3 domain. after i back odered it just 3 days later or so it had pr0 and was no where to be found on google. luckily i didnt get the domain, but does google ban the domains automatically or what?

Yes

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Thats not always true i have gotten a few domains that kept its listing ( but lost its pagerank ) so its possible the existing baclinks just made google re-spider the domain and there wa no lag time on the listing. ( the pagerank returned the next update without any additional backlinks added only the current old ones )

It usually takes some time for google to find out about an expired domain.
Getting it up fast, before it crawls the site usually helps.

I'm not aware if google gets the list of expired domain directly from the DNS structure - don't think so though