Our builder turned off the water from the water tank in the attic to do some maintenance. Since he turned it back on, the pressure in the cold water tap in the bath has been significantly lower than before he messed with it. He assures me the system is fine, but I not convinced. I think there is an air lock somewhere. How do you clear an air lock? I tried turning the water off, letting all the water drain out, then turning the water on again, but no change.