[LazyPup]

The water pressure drop that you noted is normal and is a result of the simple physical laws of nature.

The water in your house is pressurized from the municipal water main pressure. When you turn the water main off you are isolating your house from the source of pressure. At that moment all the water in the pipes is in a sealed container (the piping system) until a faucett is opened which releases the stored pressure in the piping. Water is not compressible, however their is a small amount of air trapped in the water hammer arrestors that is compressed to equal the supply water pressure. When the faucett is opened that trapped air will begin to expand, pushing the water out of the faucett until the air pressure drops to standard atmospheric pressure and the air is fully expanded. From that point their will be static head pressure at the faucett which is created by the weight of any residual water in the lines above that faucett. The static head pressure will be approximatly 1/2psi for each vertical foot of water above the faucett until all the water in the pipes above the faucett drains out.