1991 toyota 4wd 6cyl pickup. Heater core getting no water flow to it and thus no heat in truck. The core is fine and when disconnected water will pass through freely. The hose into the heater core (before the heat shutoff valve) has a drop of flow, nothing else. The engine does not overheat and I do not have a blown head gasket (runs like a champ, no frothy oil, never overheats, no loss of coolant, etc). Perhaps water in line? Water pump? (But it doesn't overheat.) Any ideas? (I've already replaced the thermostat and put a new radiator cap on.) The truck does not have any coolant leaks. I'm losing my mind!