You lucky Devil; I kept Old Pearl today, and it was sitting in the garage, still warm after an errand.
I checked the vacuum/brake thing.
At warm idle in Park, the Vacuum needle is 95% to the left, or almost full vacuum on the gauge. When I step on the brake, nothing happens - the gauge does not move. I can push really hard - nothing. When I let off the pedal, no vacuum change, it holds continually at 95%.

Sounds like you may have a brake booster issue?
If it's leaking a bit when depressed, it may not recover quickly, or may not recover completely until you blip the throttle to obtain really strong vacuum.

DG