Daves right. It will switch modes under accel and default to defrost. Its a safety default in case the vacuum supply fails. Most cars had small check valves or small vacumm delays installed after production since this is a real common issue. The black can simply stores vacuum but they leaked a lot. The cars that dont flip to defrost likely have had a small valve of amplifier installed in the last 40 years. We ahd tons of TSB's for this as did Ford and GM.
I have yet to find a black can that doesn't leak. A little black silicone around the base of the check valve in the can will cure this. Jim.....I know my canister doesn't leak....but I am unsure of the funtionality of the check valve. This is the only current "working" AC car I have. I could see if thes same thing happens on my orange car I suppose (no AC compressor installed, but eveerything else is hooked up). It is VERY NOTICEABLE on my blue car. Maybe because it is a six banger??
I have mine hooked up straight to manifold vacuum. I've got two cannisters that I bought at pull a part, a metal one and a plastic ball one, but haven't installed either yet. Mine never changes no matter where you put the selector, it's always on defrost. And I don't hear any vacuum leaks under the dash, either. I figured if the selector valve was bad, I'd hear it leaking, but so far I haven't. Haven't gotten around to installing air yet, but hope to this winter.
