Having had the same problem with a couple of older vehicles, I'd check the
vacuum canister first- one of mine had separated where the bracket was
attached. Also check any of the plastic/nylon tee fittings- they have a
tendency to get brittle and crack easily when they get older.
You can also use a spray can of ether (starting fluid) or WD40 and spray
sparingly along the suspected areas while the Bronco is running- your idle
will increase when you hit a leak.
Good luck!
   SC Tom

You may not have a leak, but rather a bad one way valve. The valve is
there to keep the low engine vacuum from pulling the vaccuum from the
HVAC system. It's usually a small disk in-line, half white and half
black (but not always that color).

I'm not sure this will apply to you, but...

Have you checked the selector switch itself?  If it's like I think it is,
it'll be a vacuum switch that directs the vacuum to different vacuum motors.
The switch is actually a valve I guess.  It rotates as you push the selector
across.

The reason I mention this is that I had the vacuum switch fail in a fan of
mine a few years ago.  IIRC, it's actually two plates of plastic that rotate
against each other with a seal between.  What happened to mine is that one
of the plates cracked and the two plates separated slightly, releasing the
vacuum pressure.  When I went to the boneyard, I noticed that many of these
had been removed already or that they were broken in the same place as mine.
I ended up getting one from the dealer.

I know there are other vacuum leaks that are more likely the source of the
problem, but don't overlook this possibility if you've ruled out others.

CJB

My guess is to check the line from engine that supplies vacum to
controls as I suspect it has a small leak in it that is okay when
vacum is high but not okay when vacum is lower because the dash pots
only need 3 to 5 in’s of mecury of vacum to work so a leak that takes
it down some from the normal 10 to 15  inchs of vacum in intake when
cruising but above the threshold level of dash pots is okay until you
step on the gas and the source vacum is  lower and the leak can no
longer be tolerated and provide enough vacum to move dash pots. Did I
lose you here?