I have a 1996 1500, 5.9, 4x2... I pull trailers and what not, but for about the past 6 months, everytime I pull more than 2000rpm (mainly uphill), the air conditioner will shift from forward facing vents to the anti fog vents. I have an idea it is a vacuum leak, but I can't seem to find where...any help?

Yes, it's a vacuum leak. there is a line that goes thru the firewall that has a check valve on it that is most likely bad.

Read this thread and it should send you in the right direction...

I had this happen on mine some time ago. For me, it was the vaccum line that runs along the passenger side frame rail right next to the exhaust. It got a tiny hole burnt into it because of how close it it to the exhaust. It cost me less then $40 and about 20 minutes to fix that section of vacuum line with a new peice from the dealer.

I'm glad I came across this post, because I have been having the same problem with my truck for the longest time....but mine does it mostly at highway speeds...I will definetly check out my vaccume lines when I get a chance..

Please post back what you find... this will help others who have the same problem.

I had the exact same problem on my 97. turned out it was a vacuum line and the cruise servo right under the battery tray. There was a pretty tight curve in it that goes into a 90 degree fitting. It was broken at the elbow going into the servo. My ac done the same thing, but i also didnt have cruise, so when i fixed that, it fixed both problems. Post back what you find. Hope this helps too.

I'm glad ya'll posted this my 98 has the exact same problem and the last two days with the heat has been a pain. Now I know what to do to fix the problem, I'll put up a post when I fix mine.

there was a service bulletin out on this to install a check valve in the line going into the firewall. My 96 did the same thing and i installed this check valve and it cured the problem.

Same problem here, guess I will have to look into the checkvalves.

Anybody know if this would effect performance as well?
My 360 wont tow a trailer that I bought from a friend with a BowTie 305, had to borrow his truck this weekend to tow it. : (

if i remember correctly, the blower switches from the face to the defroster as a safty thing. its designed so that if the vac line gets broken, you still have a defroster.