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Daves55Sedan · 10-19-2020, 06:15 PM

VACUUM for the entire system (anything that uses vacuum in the car) originates at the square brass fitting on top of the intake manifold.
Cars that have a vacuum booster on the fuel pump (dual-action fuel pump) have the main vacuum routed via metal tubing down to the fuel pump vacuum booster, and right back out to serve the accessories (windshield wipers, windshield washer and heater vacuum control system, when so equipped).
The vacuum booster on the fuel pump maintains a STEADY pull of vacuum even when the engine is LOADED, in which vacuum is decreased). The dual-action fuel pump was used mainly to prevent the windshield wipers from stopping whenever you wanted to pass a car on the highway.
Normally, your intake manifold vacuum should be around 19-20" during engine idle and anytime engine is cruising (not loaded). When you stomp on the accellerator, the carb sucks all the vacuum so that you have none for accessories, and then when you let off the accellerator, the vacuum may momentarily jump way up to around 27", then settle back down to 19" when engine is idling or cruising again.

10-19-2020, 06:15 PM	#18
Daves55Sedan Senior Member Join Date: May 2010 Location: Granite City, Illinois Posts: 3,008	Re: Heater no hot air from Heater? VACUUM for the entire system (anything that uses vacuum in the car) originates at the square brass fitting on top of the intake manifold. Cars that have a vacuum booster on the fuel pump (dual-action fuel pump) have the main vacuum routed via metal tubing down to the fuel pump vacuum booster, and right back out to serve the accessories (windshield wipers, windshield washer and heater vacuum control system, when so equipped). The vacuum booster on the fuel pump maintains a STEADY pull of vacuum even when the engine is LOADED, in which vacuum is decreased). The dual-action fuel pump was used mainly to prevent the windshield wipers from stopping whenever you wanted to pass a car on the highway. Normally, your intake manifold vacuum should be around 19-20" during engine idle and anytime engine is cruising (not loaded). When you stomp on the accellerator, the carb sucks all the vacuum so that you have none for accessories, and then when you let off the accellerator, the vacuum may momentarily jump way up to around 27", then settle back down to 19" when engine is idling or cruising again.