We were having trouble getting water out of a large water tank and I'm told it may be because of an air lock. The tank and all pipes were drained because a visitor left a tap on for a couple of weeks without turning off the stopcock by the tank. There's an outlet pipe poking up 6 inches in the middle of the tank, and a 1 inch inside diameter plastic pipe that runs downhill for 100 yards. Then there is a T junction with one pipe going up to the kitchen sink and the other going up to some handbasins. The vertical drop from the bottom of the tank to the taps is about 10 feet.
There is no other water supply available to use to force air out of the pipes.
There's several feet of water in the tank now, but none would come out of the taps. I managed to rig up a large teapot with the spout attached to each tap and sucked air out through a little hole in the lid using a vacuum cleaner. After a lot of effort and lots of water and air came out, I've got a reasonable flow of water from all the taps, but not as good as it used to be. When it rains a bit more we'll have a 20 foot head of water.
I don't really understand where the air would collect, since I imagine the pipes are reasonably straight, and a 10 foot head of water should surely push the air out. Without digging up the pipes covered in 50 year old tree roots I can't be sure what's there. I would expect the air to rise up to the taps, but apparently not. Does anyone know what was happening?