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You've got to allow air into the system to allow it to flow.
I usually do this by lifting the lever on the temperature and pressure relief valve.
If your gate valve has failed to close all the way then it's allowing water into the tank still. You may have to shut of the water supply to the building.
It doesn't take much effort.
Tell me this. You say it's flowing good? Can you stop it with your thumb? Like is it still feeling like there is pressure or is it just gravity?
Sorry I lost connection. Are you just trying to flush them out or replace them?
If you are just trying to flush them it's best to do it under pressure anyhow.
Are you still having issues?