Coolant flush Question

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Hi All, After flushing out the coolant system of my 1985 MR2, I noticed that no fluid came out of the water inlet air bleeder valve. The other air bleeder valves bled the coolant, but the aforementioned valve was dry. I probes the valve with a pen and rusty fluid showed up on the pen tip. Any suggestions about this issue? The rest of the cooling system is working great, but in full disclosure, the previous owner used water as a coolant (it took gallons of water to clean out all of the