Figure 3-15 shows how the flow of refrigerant through the gauge manifold set is controlled. The hand valve on the left controls the flow to or from the low side of the system. The one on the right controls the flow to or from the high side of the system. When either valve is turned all the way in (clockwise), we say it is front-seated.
In Figure 3-15, the Low-pressure valve is shown front-seated. In this position, both the hose and gauge above it are connected to the system so that pressure readings can be taken. However, there is no flow through the valve either into or out of the system, because the gauge manifold acts as a dead end in the hose.
When either valve is turned all the way out (counterclockwise) we say it is back-seated. Refrigerant is free to flow from the manifold into the hose or from the hose into the manifold. The direction of flow is determined by the pressure difference. Flow is from a higher pressure area to a lower pressure area.
When the valves are opened part way, they are said to be in the cracked position. The rate of flow can be regulated by adjusting the amount of valve opening. Maximum flow is achieved at the fully back-seated position.
Do not over-tighten the valves on the gauge manifold set when closing (front-seating) the valves. Over-tightening the valves may damage the manifold.