Originally Posted by nealtw
OK let's re-cap what the problem is. The problem is not related to a check valve, shut off valve or any other valve or how high the pump will pump.
At first I thought perhaps the pump pit did not have a vent, as this pit never sees a lid, that would not be a problem. Venting of the main line may be a problem as there are some requirements on pipe size and number of units being served, but the first requirements is that the discharge pipe enters the top of a horizontal pipe and with a "Y" fitting, the "Y" will have the water going the right direction. As this is hooked up with what looks like a "T" fitting, the water runs into the other side of the pipe and splashes in both directions with enough turbulence to plug the pipe.
After that is solved and then more venting still might be needed.
I know AngelaSteele5 originally said that "they" weren't avid or expert DIYer's (forgot the exact quote). So a good plumber should be able to identify this pretty easily. I think nealtw is spot on and if the plumber wasn't suggesting pretty close to exactly what he said I would tell him thank you and call another out. The hard part is finding a good one. Having had a plumber do something similar several times it's cost me around $250 to $350 for a new Y, connecting to a known good vent, and 1 other misc. change. Based on the conversation/frustration level I think under $350 would be worth the fix (I'd pay $350 to keep toilet waste out of my tub in a heart beat. As long as the project says to the Y and vent it should be pretty reasonable.
That being said it's totally possible to DIY this I'm just apposed to any DIY project that involves human excrement.