Answered Shower not draining properly

[tmac11]

This is used in drains when conventional p traps are not used. Don't ever use a plunger on that drain or you will collapse it.

Thanks - the plumbing schematics show a S-trap under the shower. I know the sinks have the flex backflow drains. Any case, once i removed the check valve on the gray tank for the backwash line, everything drains fine now with the main valve closed as it now vents the air out that backwash line at the very top of the tank! Problem solved! :0