I have searched and not found a solution for my problem. In our master bathroom both sinks and the shower have very low water pressure. If they have not been used in a while, they start out ok but then quickly slow to a trickle. This applies to both hot and cold for both sinks and the shower. The toilet appears to fill normally. The rest of the house is not affected at all. I replaced the (Delta) shower cartridge assembly and when I turned it on there was great water pressure and volume but then it slowed to a trickle again. The house is 12 years old, we have a double water heater system (run in series). The builder put in a "loop" that runs from the hot water heater drain pipe up to the master bathroom and back to the water heater to keep hot water available quickly. The symptoms I have described are the same whether the valve in the loop is open or closed. Do you have any ideas about what might be causing this?