Re: Pressure Drop
The problem is an obstruction somewhere in your hot water line before the connection where the sink/shower tee is. Possibly a partially closed valve, etc. When you turn the hot and cold water on in the sink, the cold water is actually backfeeding from the sink faucet to the shower, creating the increased pressure and the drop in temperature. I would guess that you are either dealing with galvanized piping at some point or there is a valve that is obstructed/partially closed.
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