SNRUB,
Sorry to so late responding, but I have been traveling. You are correct, normal drain at present even though water level is 2 inches above the vent pipe. And the outlet 90 does have water standing or as now, overflowing. I have examined every above ground joint and find no leaks. I wonder if the underground pipe may have cracked somehow? I will try the taping of all the above ground joints this weekend and will seal the vent pipe. Will report back next week. This has me baffled.
