I may be late here, but I would just replace the water pump while you're in there. I'm not one to throw $ away on parts, but when do you anticipate being here again?

Also, the water pump can be bad and not look it. I had a similar experience on an 84 300TD.

I would just get a new water pump and eliminate the variable. You've said it before, it's the only part that you haven't replaced. Return it, worst case.

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84 300td (Yes, a wagon)
"The Golden Dragon"
195k, 2nd owner
Lives in Redmond, WA