pipes leaking in basement

[kjames]

My home was built in 1947 and has very simple plumbing. Except for one thing. There are four down pipes in the basement (not counting the toilet and the main stack to the roof) but there are only three things that drain into the main waste line (the tub, the bathroom sink, and the kitchen sink) somewhere between the tub and the bathroom sink there is another pipe that comes down and I have no idea what it is for. This is the source of my problem, when the tub is draining, water pours out from somewhere near this random pipe. I have inspected the pipe as much as I can from the basement and there doesn't seem to be any holes in the pipe. I have also snaked the pipe from access cap at the end of the waste pipe that goes to the main pipe. The lines are clear and it still pours out when the tub drains. Where could this water be coming from and what is this extra pipe?
Thank you in advance for your help.

 

[johnjh2o]

Post a picture.

 

[kjames]

The pvc pipe comes from the tub, the second pipe is the mystery pipe, the third is the bathroom sink, the fourth(closest to the main) is the kitchen sink.
Thanks again

 

[IFIXH20]

The mystery pipe may be a dry vent that picks up the tub, lav, & kitchen then tie back into the main vent stack. You may have a partial stoppage at the san tee of the dry vent and drum trap which is causing water to backup into the dry vent when the tub drains. If you can, use a inspection mirror with a flash light to get a better look at the pipe inside the wall. Open the floor around the pipe a little for a better view.