Heyo .Welcome to the Site. It sounds like vacuum bleeding off. IDK if that's normal, a problem, a leak? Or not. Miy 90 G30 doesn't do it.
You can check lines and vacuum motors with those vacumm pump guns that they sell at HF for brake bleeding. If you need me to, I can crack a book if you need parts, service, or diagram info, but if a vacuum guage on the motor doesn't indicate a leak under the hood issue, And if the controls and heater all function normally?
Could mebe be a hose, or component, but if under the dash? I hope not. Gday!
BBC Blue
1990 Chevy G30 L19 7.4 EXT
Church Van, maternity division
DogVan
1988 Dodge B250
LA series 318 Wedge 5.2 TBI A500
Cabin Cruiser
'94 Dodge B350
www.xplorermotorhome.com/Xplorer MH model 230
230 hp LA series 360 Wedge MPI 5.9 A518 OD auto
Old vans are getting pricey!