There is a guy on another forum called dat521gatherer, he did a post about how to rebuild the heater core valves on these cores, he explained that if you remove the core from the heater box, then remove the clip near where the lever connects to the valve, then get a long stick that will fit into the hose connection end of that tube, place the stick in the tube, then tap on the end of the stick till the valve comes out of the tube.
It helps if you can support the valve end of the tube somehow, and tap very carefully, you do not want to damage the connections from the core to the tubes.
Your type of heater core removed.
Find a long wood dowel that will fit in the tube, I am using a fiberglass tube for reference.
Put the dowel in the tube till it hits the valve assembly after the retaining clip is removed and lightly tap the assembly out, sometimes you might need to squirt WD40 or some other lubricant in there ahead of time to help loosen it up, you might also need to support the tube, the valve don't always come out easy.
I don't have a photo of the valve assembly, at least not one that I can find at this moment.
The valve assembly will need cleaned up and a new O-ring installed, lubricate the assembly before re-installing it back in the tube.