Troubleshooting Vacuum Pressure
It sounds like you have a vacuum leak somewhere. Are these pumps new? If they are used you may need new vanes installed. This can cause them to have a weak vacuum. They should be drawing about 29" of hg. If they are unable to draw this with the valves closed, then that will lead you to a cause from the valves to the pumps. Either the valves aren't closing fully (there is a leak going to the pumps) or the vanes are worn.
If the valves are closed and you are able to get that much vacuum then I would start looking from your valves forward. You can have a leak somewhere if the system or the board you are using is too porous. You can try painting the edge, or edge banding the board to seal up the edges. This would prevent any vacuum leaks through the side of the spoil board. It is also possible to draw vacuum through the spoil board and your sheet. You can test this by placing a piece of paper down on top of your sheet. If the paper sticks, you are drawing a lot of air through your spoil board and sheet. You will need to use a less porous spoil board on your table. This will restrict the vacuum and hold it to the sheet.
From contributor G:
The leak might be from the gasket under the cupboard. The end of gasket is not jointed properly, or may have been loosen after placing the material. If the workpiece is small, block some area where you do not use. Just a quick idea to try.
From contributor J:
You must first get a base point to see what the pump will pull in mercury then add your sections of piping and see where you lose too much to the line. You should be able to pull around 24 to 26 inches of mercury with two pumps.
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