Which warning are you receiving?
Every other second....... Every 20 or 40 seconds....... Constant and steady sounding?
If simply air in the line due to disconnecting a oil line or whatever, the VRO would require priming as follows:
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(Priming The VRO Pump)
(J. Reeves)
When hooking up or installing a VRO whether it is a new or original VRO pump, it must be primed in order to dispel any air that might be in the oil line.
Have the Oil line attached to the engine fitting BUT detached from the VRO. Add a piece of fuel hose if necessary to the oil line so that it can be aimed into some sort of container.
Pump the oil primer bulb, catching the oil flowing out of the line into the container until you are absolutely sure that you have rid the oil line of every bit of air that might have been trapped there.
Now, connect the oil line to the VRO and secure it. When the oil line is secured, apply pressure to the oil primer bulb ONE TIME only. That's it.
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If after priming properly, the "No Oil" alarm (every other second) is still encountered, and it goes off when the oil primer bulb is compressed... BUT... comes back on shortly thereafter, the VRO requires either rebuilding or replacing.