Robinair Model 15600 - Manual (Page 5)

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To use the gas ballast feature...
Moisture from the A/C-R system that is carried into the pump as a vapor
tends to condense into a liquid and combine with the vacuum pump oil.
When moisture contaminates the pump oil, it reduces the pump’s ability
to reach its ultimate deep vacuum level.
The gas ballast valve purges a small amount of atmospheric air through
the exhaust chamber. This extra volume of air mixes with the vapor
from the refrigerant system to prevent condensation and to help exhaust
moisture in the form of vapor from the pump.
The gas ballast valve is located beside the handle, opposite the inlet
fitting. To use the gas ballast, start the pump and open the gas ballast
valve until the system has reached 1000–3000 microns. Close the valve
to allow the pump to pull down to its ultimate vacuum level.
The gas ballast valve may be opened or closed at any time during pump
operation. It is fully open at two turns counterclockwise.
Note: Robinair recommends the use of a thermistor vacuum gauge to
most accurately measure vacuum levels.
To shut down the pump after use...
To prolong pump life and promote easy starting, follow these procedures
for shutdown:
1. Close the manifold valve between the pump and the system.
2. Turn the Iso-Valve™ to the CLOSED position.
3. Remove the hose from the pump inlet.
4. Turn the pump power switch to OFF. Return the Iso-Valve™ to the
OPEN position for a few seconds to relieve any vacuum inside the pump.
5. Cap the inlet port to prevent any contamination or loose particles from
entering the port.