[Shop-talk] Smoke gun for septic vent leak?

[Jim Franklin]

Apologies if I've asked this before, I can't find any emails from it. I rent a basement apartment with a sewage ejection pump. When the weather gets cold, it stinks, almost exclusively while I'm in the shower. After many investigations and lots of thought, I think there's a leak in the vent side. As the catchment tank fills up, the air above the liquid can't push the cold slug of air in the vent pipe, so it exits through the leak. 

A plumber has been here 3 times, and yesterday replaced the noisy and banging backflow prevention valve, but either he's a nancy boy who doesn't wnt to dive any further down the waste side, or is clueless, because his excuse is that it's very hard to diagnose odors. Or maybe he's doing it for free as a friend of the landlord and doesn't want to create an all day job he won't get paid for.

I'd like send a large volume of visible air down the vent and see if it leaks out somewhere. I'm thinking I'd drill two small holes- one for a balloon which I'd inflate to seal the vent, and one below it for a smoke gun. Sound plausible? Better way to do this? Is there such a thing as a high volume smoke gun for consumer prices?

I've tried soapy water and smoke sticks to look for escaping air, but it was inconclusive; I think I need positive pressure inside the system. I suppose I could install a coupler in the vent and use the compressor and aforementioned smoke sticks, but I'm looking for a better plan A. 

thanks,
jim