I have been in my house for about 2 years - it was a new build. Since it was first build there has always been intemittent sewer smells eminating in the various bathrooms. There are four bathrooms - the main one on the first floor is connected to a soil vent pipe that exhausts through a roof vent. The first en-suite is also connected to another soil vent pipe that used to exhaust through a roof vent - it now has an AAV fitted as this room often had the most consistent smells and it always seemed like it was coming back through the bathroom extractor that was always quite drafty. There is also another en-suite and a cloakroom below it that are connected to a further soil vent stack that is capped in the loft with an AAV.
Over the last week or so the cloakroom is smelling like rotten cabbage and at worst like a sewer. This evening I have dismantled the pipe work from the small sink in the cloakroom and the standard pipe is fittted through a flexible rubber adaptor into a standard 100mm drain pipe bend. To my surprise there is quite a positive air pressure coming up from the drain and I suspect that this will be driving the smell through the weakest joints - I only fitted the AAV to the other stack last week. What do I do now, is it worth capping the only remaining vent to air with an AAV as the houses adjacent can provide the air exhaust for the drains. I don't want to do this if it makes it worse.
Any suggestions or similar experiences would be appreciated. I can normally fix anything but this one has got me baffled.