Try this stuff called Thrift. Got it at a plumbing supply store for about $7 for enough for two treatments. Stuff is safe for plastic pipes and septic tanks and works in just about a minute. I used it in my home bathroom sink with PVC pipe after Drano did nothing. It's basically straight lye. (Don't hold your face over the drain when you use it--the chemical reaction is vigorous!)
http://www.thriftmarketing.com/thrift_drain_cleaner.htm
LOOK in the vanity cabinet where the sink is, usually a 2 inch pipe with a funny looking cap on it that seems to be running nowhere. that would be your internal vent if one is not going through the roof. take off the cap and check for proper operation or replace.
Bill
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