Bob, << Have a vacuum gauge on top of Racor 500 filter. Gauge seems to have bubble in it near top of gauge face and very slight weeping of fluid around outer metal rim (where glass face is retained). Gauge does not seem to read accurately. What is happening and how to fix? -- >> The liquid is glycerin and is used to dampen (slow down) the needle movement. When the pump actuates, there is a pulse of increased vacuum, the glycerin causes that to average out so you can see the needle. There is a little rubber plug in the top of the gauge, pull it out. Mine came with instructions to snip off the tip of the rubber plug to allow it to breathe while still keeping most of the crud out. When you put the plug back in, the gauge should read "0" again, assuming you have no vacuum on the line. Wayne M/V Celestial Albin 43 Sundeck