Stinky Tub Drain

[RustyPipes]

We bought this 1950s house from the first owner. It still has the original plumbing. (mostly)
The vents are very poorly designed. The problem is the tub drain. When we were living here for several years the tub trap began to leak. I found that the trap was lead and slightly folded between the tub drain and a floor joist. The connection between the brass drain of the tub and the lead was corroded. Eventually, I replaced the lead pipe with a plastic trap. Because of the proximity of the joist, I turned the trap at an angle since the plastic trap would not fold.
After some time the drain began to emit a rather horrible odor. When we clean the trap, it is full of a black slime and human hair. I don't remember any odor or trap clogging when it was lead.
Back to the vent comment. Before the trap started leaking we noticed that when the toilet was flushed the water was sucked out of the tub trap. I installed a "NSF" vent between the toilet and the tub which cured the loss of the water in the trap.
The bathroom pipe arrangement is Sink-vent-toilet-NSF-stack-tub.
Since the kitchen ceiling had to be cut to install the PVC trap, I don't want to experiment.

Any recommendations?

 

[Smooky]

NSF stands for National Sanitation Foundation and it is a product testing organization. If a product has the NSF seal it just means it meets standards for public health protection. A lot of different plumbing and other items have an NSF certification. I'm guessing maybe you are talking about an air admittance valve (AAV)? Have you got any pictures of what you are talking about?