If the water comes out black, then it probably hasn't been drained/flushed too much. I've opened some where it comes out like oil for over an hour. I know that whoever was supposed to be maintaining never did it in the years he was there. Some come out a little rusty for a bit then clear up.
It can be either a dry or a wet system.In a dry system, it's under air pressure. When there is enough of a pressure drop, a clapper (check valve) opens releasing the water.
In a wet system, it's filled with water.
Being that they were indoors, I'll assume it's a wet system. Dry systems are...