Why is the holddown power is decreasing on my vacuum system?

From Shopbot Wiki

Vacuum loss over time:

"I was consistently getting 12-13" when my trupan spoiler was new (18mm to start, then skim both sides). But as it got thinner over the months, I noticed a decrease in holding power and reading at the gauge. Now the board is ~1/4" and the max I can get is 6" of vacuum. The pump is not bad, as I can shut off all the valves and still reach the 14" magic number, and the filters are clean, gauge upstream of the bypass reads the same as the gauge at the table. The spoilerboard seems to be too porous now that it is thin.

One of the shops that used to cut parts for us used a 3/4" spoiler with 1/8" top sheets that they would just toss when they got chewed up. That way they never bit into the 3/4". Might try that too. Need to see how much a stack of 1/8" mdf runs.
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