My 02 F350 has had an issue ever since I bought it about 3 years ago. The vacuum system doesn't seem to have enough power to keep up with the Max A/C setting. It works fine (except that switchovers take forever) everywhere else on the dial, but when you switch it to Max, it will start to blow out of the mid-level vents, then slowly revert back to the defrost vents. I've just lived with it as there have been a million other problems on the truck much more important, like worn out ball joints, and steering joints.

I have also noticed that the vacuum pump seems to never shut off no matter where the A/C dial is set, so I thought "Aha! A vacuum leak!" So tonight, I opened the hood, turned the key to "run", set the A/C to Max, and started tracing the vacuum system starting at the pump. I pulled the intake hose off and plugged it with my finger, and the pump shut off almost instantly. Next, I pulled the hose off the other side of the vacuum reservoir and plugged it with my finger. The pump kept running and running and running and running. I counted to 274, lost count, and counted to back to 42 again but the pump kept running so I thought "Aha! The vacuum reservoir is the culprit!"

So I pulled the reservoir out and decided to test it on the bench. Now I am far too cheap to own a HandyVac or whatever they are called, so instead I totally wrecked the reservoir with compressed air. I put a blower nozzle on the nipple that goes to the pump and without thinking gave it a quick spritz of air. I heard the unmistakeable sound of a checkvalve flying apart and ricocheting around inside the reservoir. OOPS! I figured that it was already wrecked, so I might as well continue on with my experiment, so I soaked it in soapy water, plugged one of the nipples, and applied a slight pressure to the other one. Not a single bubble anywhere. So it turns out I probably have a weak pump, but now I have to replace the reservoir also.

Moral of the story: "Use your noodle, you damned cheapskate, or you end up spending more money than if you do things right in the first place!"

 

DOH! Its ok, we've all been there.

Get yourself a $20 air pressure regulator for that compressor. That way you can test the system when you get a new reservoir in there. Plus they are also just nice to have. I have one for using on a boost leak detector.