My tub drain is downstream from both the toilet and the roof vent and every time the toilet flushes the tub trap is emptied and I get sewer smells...
Photo @ www3.ns.sympatico.ca/dlpotter/Plumbing/
I think I need to add an anti-siphon device between the trap and the cast iron.
Comments/suggestions...

Thanks,

David

Well, an AAV would probably help, but it isn't going to be easy to install.

You would need to cut out the lead trap altogether and then remove the caulking ferrule from the 2" hub -- When the hub is completely cleaned out, you would then use a Fernco Tye Seal, a short piece of 2" pipe, a 2"x1-1/2"x1-1/2" Wye, a 1-1/2" Street 45, a 1-1/2" DWV p-trap, an 1-1/2" trap adapter, a foot or so of 1-1/2" pipe and an AAV.

The hardest part would be getting the Tye Seal and the short piece of 2" pipe into the cast iron hub, especially in such a confined space.

Personally, I'd suggest handing this off to a Plumber who knows all the tricks of the trade.

Just my two pence.