When the low pressure pump starts and water comes out the thru hull on the
port side that indicates that the pump is putting out water flow. The water
maker will make water and the high pressure pump will usually operate ok
WITHOUT the low pressure pump.
However, the high pressure pump is sensitive to cavitation or air bubbles. It
isn't very good at sucking water, just pushing it. That is why you have the low
pressure feed pump to help prevent air bubbles entering the high pressure pump.
At any rate it sounds as if your low pressure pump is operating ok and I believe
that you have an indication problem. Either the gauge is bad, plugged, or the
tubing to the gauge is plugged.
I suggest that you undo the B-nut on the back of the low pressure gauge that
attaches the blue tubing to the gauge and check to see if you have any
obstructions in the orifice that enters the gauge. If it is clear then place the
blue tube into a bucket to catch any water and start the low pressure pump
in the normal fashion and see if you have water low thru the blue tubing. If
there is no flow then check the input side of the blue tubing to see if there is an
obstruction. Just break it down into its individual parts and trouble shoot
An air bubble in the blue tube would not cause the indicator error you are describing.
Air, while it is compressible, will still transmit pressure to the gauge. It might
dampen the reading but it would not cause a zero reading. You can check the
gauge using compressed air by rigging up a hose to apply regulated air pressure
to the input nipple of the gauge. Before you do that do the simple things noted
Best of luck and let us know what you find.