Plumbing Question - Sinks won't drain

[Geode Pratt]

Hi All,

This may be bit off topic, but I know the experience in the room will have answers.

We're almost at the end of a flip rehab and just realized that the master bath sinks (dual vanity) won't drain - rather, fill quickly and drain VERY slowly. Maybe an inch an hour. Meanwhile, the tub and toilet, which are about 6-8 feet away both function perfectly. Both vanity sinks have the same problem, and they appear to "T" in the wall, though I haven't opened it up, as the cabinets are painted, granite is in, etc. Unfortunately, the p-trap is glued PVC, not screw in, so even opening that will be a bit more of a pain. Anyway, we snaked both sinks 15' from the drain and found nothing. The only vent stack in this bathroom goes directly to the toilet, so unless they're all connected in the bulkhead, (I have not yet dug through the attic insulation) the sink may not have a vent.

That's where this goes above my head.  

My concern is there may be a break in the sewer line somewhere under the slab, but it would seem that would cause other symptoms besides just these sinks. I'm about 24 hours from calling a plumber, so I'm hoping the fine folks of BP have a solution.

 More info: It's a late 70s build on slab in Texas. Not F&J, but a step or 2 above. 2000 sft brick, beds/baths on one side - kitchen on other. No side cleanout on this side of house, but one by kitchen.  I think 2 cleanouts in front yard by curb.

Any help on this is greatly appreciated!