How Refrigeration tools work
A Refrigeration Vacuum Pump is modified in three respects. The bypass line which runs between the charging house and the check valve must be cut off and the ends sealed. The pump will then produce a vacuum of one millimeter if the check valve is open and the strainer is not wet with oil. The check valve opens automatically when the pressure in the system is above three millimeters. The pressure will not drop below 10 millimeters if the strainer is wet with oil. Oil can be removed from the strainer and the check valve opened by permitting air to flow through the pump for a few minutes prior to connecting the unit to the vacuum system. The screen may be removed, but great care must be exercised thereafter to prevent dirt or foreign material from entering the pump. To make this modification, cut the intake line about an inch away from the charging house. Use a tube cutter, not a hacksaw, or particles from the saw will almost certainly find their way into the pump and cause it to stall. Bend the cut tube out of the way, then dig the strainer from the opening by means of a small hook made from a nail or steel wire. Inspect the opening carefully and remove all stray wires of the screen by means of tweezers. Cap the opening with a short length of rubber tubing and close the end with a pinch clamp.