The heater valve regulates flow through the heater core.
The radiator cap regulates pressure of the cooling system.
Early Chevy cooling systems had zero pressure radiator caps.
The later 4-7 lbs. caps raised the coolant boiling point.
I once had both a radiator and heater core that only leaked when coolant was under pressure.
A zero pressure radiator cap reduced the leaks, until I could fix them.
