Whenever we wash dishes in our double sink in the kitchen, the water in our bathroom toilet, sucks out, all the way down to the toilet trap. Obviously they share the same vent, which also has the bathroom sink hooked to it. All seem to drain ok ecept for that one problem.The vent stack looks to be free of any blockage. I have never made any major changes to any of the plumbing system other than change two 2" pipes in the wall running from the bathroom and kitchen sinks to the main line that runs outside. I sloped those pipes slightly towards the exit. We live in a ranch house that is on a slab.foundation. Any suggestions would be greatly appreciated! THANKS MUCH, in advance.