Sewer vent stack leaking into basement

Hello. We have been having a problem lately with one of our sewer vent stacks. It is at the back of our house and attached to our kitchen sink then continues straight down to the basement. The trouble is that it leaks whenever it rains. It doesn't leak a lot but it's a huge problem because it is right next to our electrical panel. We thought at first that it must be a roofing issue. We had a roofer come over and look at it and nothing appeared to be wrong but he suggested we seal around where the metal meets the roof. We did that and there was no change. Finally we did a test where we sprayed water on the roof and checked for the leak. No water downstairs. Then we sprayed some water directly into the pipe and that's when we saw the water downstairs within a few seconds. So we know for a fact that it's the pipe. The problem is we can't figure out where the leak is or how to fix it. It has to be somewhere above the sink upstairs right? Otherwise it would leak when the sink drains which it does not. It is almost impossible to get over to it from the attic because it is quite a distance and the attic is not high enough. We would have to crawl over there. If it was leaking in the attic would we see damage on the ceiling somewhere? My husband is thinking about buying a chimney rain cap for it. Would that work? Is it safe to do that? We would take it off every winter since we don't have any issues in the winter. Thanks for any help!

 

Not the plumber but pictures would help.
http://www.doityourself.com/forum/el...-pictures.html

 

some pictures of the stack from outside and two from down in the basement.
Also a picture of the sort of piece I was thinking about buying.

 

I had a similar problem in my house. It turned out that one of the joints was not glued. I was able to find the un-glued joint in the attic and glue it properly. With your's located in the outer wall you have no room in that attic. You could try a cap like you pictured or glue on a couple 90 degree elbow fittings so the pipe makes a 180 and the opening is facing down. Since you are in a snowy area I would extend the vent up higher before trying the 180 fittings so the opening can remain above snow on the roof.

 

I agree with Pilot Dane, I was thinking a U bend would work best and easiest. Since it's all PVC, there's probably just a bad joint, probably near the top. It would leak more often if it were further down the stack. Also, it's unlikely split like it might be if it were cast iron.