Remove the valve stem caps and gauge connection caps from compressor discharge, and suction valves, and connect your gauges to the valves(compound gage to the suction, and high pressure gauge to the discharge valve). With the help of a service wrench, turn the discharge valve stem all the way clockwise, and crack open the suction valve gauge port by turning the suction valve stem 1/2 a turn clockwise, and turn on the compressor and observe your low pressure gauge. The compound gauge reading should drop to 29″ inches of vacuum within 60 seconds or so. At this point turn off the compressor. If within this period, the reading on the compound guage begins to move toward zero, the compressor has an internal lear (Bad Gasket, O’ Ring, Bearing, etc.), and must be replaced (only bolted type compressor are repairable (Figs. 119, 120) if the compound guage reading never reaches 28″ or 29″ of vacuum while running, it has lost its pumping ability, and must be replaced. An efficient compressor must reach about 29″ or vacuum, an maintain the level of vacuum after it is turned off.
Residential refrigerators and freezers that do not come with access valves can be tested for efficiency by pinching off the liquid line (see Figure 45g) by using a pinch off tool, and by installing a tap value (see Fig. 45f) on the suction line before you follow the same procedure.