As others have suggested, this might have a drum type trap and that may be
where I'm having difficulty getting past. Anyway, I used some rubber
washers and the vent plate to seal off the vent and forced some pressure
into the drain. The drain is working but slow, at least it's useable. I
plan to see what I can do with some chemical (vinegar, bleach, ???) and hot
water. Other than that my only plan is to try to save up $$ to have the
drains pipes replaced with snake friendly traps. Maybe I'll come back with
a question on installing 4" drain pipe in a sink! :-) At least since it's
flowing I can get some chemical down there that's not diluted in gallons of
water.

Any recommended chemical treatments for hair/soap scum clogs?

Thanks!

RogerN