Air stops blowing out vents pulling a hill

i think you are on the right track. my guess would be that the switch may need to be replaced.

Mine does something similar. I will have the A/C on, and it will sometimes randomly switch to defrost. Hitting the brakes hard a few times switches it back most of the time. I think this started happening after I installed my new double din CD player. It hasn't done this in a while, but I would still like to know the cause.

I can think of a couple of things to check.

1. Check the under-hood vacuum line to make sure there is no break. The line is rigid, but meanders around and on some vehicles goes to a "block" where several lines are attached. If anything is broke or cracked, the drop in engine vacuum going uphill will bleed the vacuum away and the vent doors will usually go to defrost only.

2. There is a vacuum reservoir with a check valve on both ends. One side goes to engine vacuum, the other goes to the A/C controls. This reservoir is supposed to hold vacuum while you are in a low-engine-vacuum condition to prevent exactly this. If the check valve goes out, the vacuum will not hold when engine vacuum drops and you see this problem.

3. the a/c switch on the dash opens/closes various ports to route vacuum to the right vacuum controls to open/close the proper doors. If this switch develops a leak, it will also let the vacuum bleed down when you are in a low-vacuum situation.

A simple mighty-vac type vacuum pump can find the leaks if leaks are present... Your finger will do well as you unplug lines and use it to see if there is vacuum present...