What is the vacuum valve on return side of heater hose a 69 Chevy 350 for?

The vacuum valve on the return side of the heater hose in a 1969 Chevy 350 is a temperature control valve. It is designed to regulate the flow of hot coolant from the engine to the heater core, allowing you to adjust the temperature of the air coming out of the heater vents. When the engine is cold, the valve opens, allowing more coolant to flow through the heater core and providing heat to the cabin. As the engine warms up, the valve gradually closes, restricting the flow of coolant and preventing the cabin from overheating. This vacuum-operated valve is controlled by a vacuum signal from the engine's intake manifold, which varies based on engine temperature.