Manifold Hand Valves
The purpose of the manifold is to control refrigerant flow. When the manifold gauges are attached to the system, pressure is indicated on both gauges at all times during normal conditions. During testing, both the low and high side hand valves are in the closed position (turned clockwise all the way until the valve is seated). You will note that even with the hand valves closed, the high and low side pressures are still indicated on the respective gauges. That’s because the valve stems do not prevent refrigerant from reaching the gauges. They simply isolate the low and high sides from the central service hose port on the manifold.
When both hand valves are closed or when one or the other is open, the low and high side gauges will give good readings. However, when both valves are open, gauge readings are not accurate because high side pressure escapes into the low side of the manifold and influences the low side gauge reading.