Here is something I will describe.. You have a "Water use reducer" washer this is a small washer with a tiny hole (lets make it a pin hole) in it in a line, When you shut off the valves a pressure gauge on the down-line side of the thing shows full line pressure.
but when you open a valve the pressure drops to zero and water basically drips out of the faucet.
I have actually seen this in electronics, the DIGITAL Volt meter showed normal voltage because it used less current (water) than the fault (Washer) would pass.
But a test lamp found it and fast.. (And the fault (washer) was thus removed and full flow restored).
