Been there, done that in the shower. I'm sure that Brian Woods product works well, but will not stop your problem (sorry Brian--and I don't mind hearing about new products). How you can tell this is easy enough. Just be taking a shower and have someone flush the toilet. Then you can scream. What happens is that the cold water goes to the toilet and the cold water pressure drops in the shower. The hot water pipes go on doing their thing. Mike is on track on this. Henry in MI