After months of the familiar our individual fixture traps testing I have finally located the entrance of Drain flies into my house. They are coming up from a crack at a concrete slab and a cinder block wall which is behind a partition wall. This crack is about 18" away from the 3" verticle cast iron vent stack that goes through the roof. I am 100% positive that there is a galvanized drain line (which I strongly suspect has a crack on the underside) from the kitchen sink (60 65 years old) running under this concrete slab tying into this vent stack about 24 to 30" below the slab. This stack drops about 2' down into a sweep, thru the exterior block wall and then ties into a newer PVC line under a newer slab of an addition that is about 25 years old. We have used cameras in the newer PVC main 3 and 4" drain lines with no evidenbce of a blockage or cracked pipe. The furtherest point the camera could go was at this sweep just beyond where the cast meets the new PVC. The camera people said they could not use their camera in any smaller lines. The camera people said that there was evidence that water was flowing freely in the galavanize pipe. Is there any camera available that can be used in a 1 1/2" or 2" galvanized line? Is there a way I can be totally sure that the home base of these flies is located at or near this galavanize drain line and the vent stack without extensive demoli8tion and digging in this area which is totally in the interior portion of the house?

They do make cameras that will traverse a smaller pipe, but it isn't easy to get them down anything smaller than a 2" as the turns tend to be nearly impossible to push the camera through. If you call around to different plumbers, someone probably has the smaller one as well.