[VAC] Help--A/C while towing

[Dan Weeks <theprofileco@xxxxxxx>]

I regularly use the A/C while towing in my 86 Caprice 305 4V, and have no
problem keeping coolant and trans oil temps cool. The car interior is cool,
too--except when manifold vacuum drops to near zero in headwinds or while
climbing hills, the vacuum-operated air doors in the HVAC system default to
defrost mode, blasting the hot windshield with cold air instead of the
passengers. I've noticed other vehicles do the same thing when running wide
open for any length of time. My brother the mechanic speculated that one
could install a check valve in the vacuum line to the air doors. Has anyone
ever done this? Know what parts to get, from where? Or is there some way to
just jury-rig the system to stay set on using the dash a/c vents until
summer is over? 

ALso, am I right that at full throttle, the compressor automatically is
clutched out for maximum power to the wheels? If I go from zero vaccuum at
full throttle and actually hit the kickdown switch, I don't even seem to get
cold air.

Dan
75 Argosy 26
86 Chevy Caprice Classic Wagon, 168k