I have a 1992 model 240 and it has a vacuum leak that seems to be coming from the recirculation line that runs from the heater control panel out to the intake gate. I know this because when I push in the recirculation button in the heater control panel, this shuts off the vacuum pull from the main vacuum line into the recirculation line, and the hissing sound stops.
When the button is in the 'out' position, vacuum is pulled through the line which runs from the button to the intake gate on the other end of the line.

Locating the intake gate on the other end is what I am interested in.
The owner' manual says that when the button is in the 'out' position, I'm getting full flow of outside air. That tells me that the vacuum being applied when the button is out is holding open the intake gate which must be spring-loaded, thereby allowing air to flow in.

What I would like to do is to locate this intake gate at the end of the vacuum line, as I'm wondering if there is a loose connection there which is the cause of the vacuum leak. Or it could be that there is a hole in the line going to this gate, in which case I could disconnect and replace this line.

Does anyone know where I might find this intake gate?

I thought at first that it might be in the engine compartment, but now I'm wondering if it could be under the grill somewhere just outside the windshield.

Thanks.