I drained my water heater yesterday (1 year after installation). I turned
off the cold water supply into the water heater, turned off the gas, open a
hot water faucet on the second floor, and then open the drain valve on the
water heater.

About one gallon of water comes out, then no more. I have to open the
pressure relief valve to let the rest of the water out. Why is this?
Shouldn't the open faucet on the second floor break the vacuum and allow all
water to come out?

BTW, the water being drained out was clear with some black specks floating
in it. I was expecting murky water.

Anyone knows how to adjust the air pressure in the thermal expansion unit on
the water heater? Or at least check if it is correct? The manufacturer's
instruction only says to hire a plumber

http://www.amtrol.com/thermxtrol.htm