a gauge manifold to the receiver outlet service valve, with the valve in the frontseated position, so that the receiver is blocked but the gauge has access to the liquid line. Refer to Figure 15‑19. When in the pump-down mode, repairs to the high-pressure side can be accomplished only after recovering refrigerant trapped in the hot gas discharge line and condenser. The recovery process is performed at the discharge service valve. See Figure 15‑20. needs to have a vacuum pulled before the receiver service valves are opened and the system restored to service. See Figure 15‑19. If the low-pressure side is left open to the atmosphere for too long, atmospheric air will enter the system, and vapor will leak back through the compressor valve reeds. The atmospheric air must be removed with a vacuum pump prior to releasing refrigerant from the receiver. Also, the lost refrigerant must be replaced. Evacuating air from the low-pressure side is done by connecting Intake Exhaust Vacuum pump Frontseated Frontseated Closed Frontseated Open Goodheart-Willcox Publisher Figure 15‑19. If the system is left open to the atmosphere, air will eventually enter. The evacuation process for removing air from the low-pressure side is shown. 276 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.