I'm rehab'ing an 86 I bought a while ago. during the process of fixing all of the vacuum leaks due to aged hoses, I've run into one can't find and suspect is causing a performance issue. I have checked all over the engine bay and all hoses are secure now. This leak is somewhere in the dashboard. I can hear a de-pressurizing sound when the engine is turned off. As well, at around 55mph and under some load, there is a high pitched whistle coming from the dashboard.

The performance hit is a momentary lack of response to change in throttle. I'd like to eliminate this obvious leak before trying to troubleshoot other systems.

I checked the two rubber lines that run the the expansion/vacuum tank under the heater core area. Both of those were in good condition with no cracks. I tried to trace them but quickly get lost in the thight space. I also have the stereo out (mounted up top), so I can see in above the center vents, but the limited visual inspection was a bust.

Never having had to replace the blower motor before, I would rather not take the dashboard apart more than I need to. Any standard gotchas of the vacuum system within the cab?
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'82 245 B21F-MPG-LH 1.0, '83 244 B21FT-K-Jet, '86 244 B23F-LH 2.0