A quick fix is to clamp the heater hose feeding your heater core (and unplug the power to the circulating pump) to overcome the malfunction of the heater monovalve, which has lost its vacuum source due to some sort of leak. Don't use this fix for long because the hose could get bruised, etc.
I can go into more detail on vacuum testing this system and a more satisfying temporary fix, as I just had this problem. Cheers, Andrew Seidel.
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