[Christ]

It's very simple... and it might even work.

If you could design/build an electronic switch, a MAP sensor would also work, but you'd need an Arduino or something similar to read the signal and control the circuit.

Basically, set it up so that the MAP sensor's reading controls a switch. Under 16" of vacuum, turn on, over 18", turn off.

You take an old pill bottle, drill a hole in the top of it, insert the MAP sensor (which needs a 12V/ground as well as a sig output) then drill another hole for a nipple, to which you would attach your signal hose. Run the signal hose in a Tee with the bag hose, so they "see" the same vacuum both inside the bag and inside the pill bottle.

When the pill bottle's vacuum reaches 18", the Arduino kills the circuit (the pump's circuit - leave the MAP turned on) and when the bag falls below 16", the Arduino turns the pump circuit back on.

This way, there's nothing mechanical to align, nothing can slip, break, or otherwise fault, and you can build in redundancy that if the system drops below a certain vacuum level for X time, to turn off the circuit.

Say, for instance, a hose cracks. Pump starts pumping, because vacuum is below 16"... pump can't get anything higher than 10" after 30 seconds, Arduino shuts down pump to prevent damage.