How much vacuum is there ahead of the throttle body? Enough to get a real vacuum reading on a gage? Atmosphere is about 15 PSI. If say you had 2 PSI with a new filter, pitch it when it got up to 3?
If the vacuum was less, it would be more accurate to drill a slightly undersize hole in the plastic duct work near the top, and force a length of small diameter clear tubing into it. Keeping it way from heat and mechanical hazards, let it loop down to near the bottom and back up. Fill it half full with coolant, easier to see and won't freeze. With the engine idling, mark where it stands. Juice the throttle and let off and see what happens. I guess if all the coolant is sucked out, you may have decarbonized the engine even if it didn't need it. Maybe start with pure water until you see if it is going to be sucked into the engine.
If the upper level of water stands 2' above the lower, that would be about 1 PSI. Keep an eye on the levels. If the difference goes up much, the air filter is becoming more restrictive. If it goes down, check for leaks. You might get a more meaningful reading at a higher airflow.
This is just an idea I am throwing out. Maybe it is an idea that well deserves to be thrown out permanently.
