toilet backs up when black tank 1/3 full
When my black tank is empty the toilet flushes just fine but once I get about 1/3 full I start getting bubbles coming up when I flush and at 2/3 full it is filling the pipe. I can break this apparent vapor lock by applying suction with a plunger and pulling air out of the the tank but the victory is short lived. I took it to the dealer and they cut the vent pipe and inserted a bore scope to assure patency. I have opened this divide(the repair was just a rubber slide over with clamps) just to see if that changed anything and it didn't. In fact, I still had to flush the toilet to get enough air flow to make it drain.
My last experiment was to put an electrical wire fish down to get a feel for the geometry and it seems that there is only an inch or so clearance between the end of the pipe and bottom of the drain. I don't know how deep the tank is but I suspect that this is my problem. Has anyone else experienced this? If so, how did you solve it?
Chapparal lite - 266Sab