We are having a problem with out upstairs bathtub draining. When my daughter takes a bath and the water is drained it goes out the main waste line overflow. This does not happen when we shower in that bathtub and it does not happen with any other drain in the house.
Last year we had a new kitchen installed (below that bathroom) and they had to redue the plumbing and move the drain pipe up. I am thinking with the new angle of the drain it can't handle the water. However, it does not happen every time.
Any suggestions?

Can you better describ what is happening with "it goes out the main waste line overflow. " I am having trouble trying to picture just what you mean.

We have a overflow vent on the front of the house, this is where the water comes out. The waste water does not make it to the cesspool.
Let me know if you need more info
Thanks

Your main sewer line is clogged. That outlet on the outside is not a vent but a clean out. That clean out should have a plug. Have your main sewer pipe snaked with 3/4" snake all the way to the street. When finished, as the attending plumber to install cap on the clean out. It is health hazard not to have one.

Thank you for the help.
But if the sewer line is clogged wouldn't I have a problem with all of my drains?

Good question...

Conventional bath tub holds 40-80 gallons of water (depending on model and type of tub ). When you drain tub, all that water fills sewer line rapidly and continuously. Partially blocked sewer is unable to drain all that water through the blockage so it starts backing up, overflowing through the lowest outlet in your system - in your case - the uncapped clean out.

Other plumbing fixtures in your house deliver only small amounts of water - not enough to cause back up. Sinks deliver about 2 gallons a minute, shower 2.2 to 4 gallons a minute, toilet 1.6 gallons a flush and washer (depending on model) about 4-16 gallons in 45 seconds. It is not enough water to cause overflow.

One last question, I hope.

If I have the clean out capped, wouldn't any overflow back up into the house? As of right now the bath water comes out the clean out on the front porch, which seems a better scenario than having the waste water come back into the house.

Thank you

True... with clean out open it will overflow into the back yard rather than inside your house.

But clean out is there Not relieve excess of sewer but to access sewer line in case it needs to be cleaned. Additionally, uncapped clean out is health hazard. It is Code violation to have uncapped clean out.