I just bought an old farmhouse (really good deal) an am going through
all the repairs that need to be done and now have a puzzle.
When a large amount of water is flushed down the pipes (i.e. bathtub
emptying) the basement fills with sewer gas. Found a clean-out cover
rusted out (and now fixed) but still having the gas odor.
I think that when the waste water enters the tank, the gas gets pushed
back. After the fix it has dropped noticeably, but it appears it has
now been directed to the tank vent.
There's still some gas in the basement, and a fairly good stench from
the septic vent, have I been living in the city so long to forget that
this is normal?

What do you mean by the "septic vent"? Normally, your main drain line
should have a vent pipe that runs all the way up to the roof. There should
also be individual vents for each fixture (sinks, toilet, tub, etc.).

Some older homes may not have vents for each fixture. This creates a vacuum
in the drain line when you dump water down the drain (like the glug glug
you get when pouring water out of a bottle). For something like a sink
there's usually still air space in the drain pipe that allows some venting.
But draining a bathtub may completely fill the drain pipe, so it will be
trying to suck air from any place it can in the system.

If the vent extends through the roof, but you're smelling gas in the
basement, you have a leak somewhere. If no water is leaking out anywhere,
it's probably a leaky vent pipe.

You might try applying some really soapy water around all the joints in the
pipe and see if the soap bubbles up anywhere when you drain the bathtub.
Though in theory, it should be sucking the soap inward and not exhausting
gas out.

Assuming you have a roof vent, you may want to check that it is not clogged
with a bird nest or something. I recommend getting a cap for the top that
allows air to enter and gasses to escape, but keeps out the birds
(available at any home center).

Good luck!

Anthony Watson
www.watsondiy.com
www.mountainsoftware.com