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A/c stops working going uphill

A/c stops working when I am going uphill even if it a very slight uphill. No air comes from vents.

You have a bad vacuum check valve. The check valve is there to make sure the vacuum to the vents is maintained when the engine vacuum drops, like when you accelerate up a hill.

Bad vacuum check valve or a small vacuum leak somewhere in the climate control system. If you check, you’ll find that the missing air is coming out of the defrost outlets when this happens.

On the 20-year-old car I recently sold, I would loose defrost air when driving uphill.

The check valve on that car was easy to spot - right on top of the intake manifold. Washing it with hot dish water every 6-8 years put it right.

Right you are. I bought a used Olds Silhouette. The previous owners explained the A/C didn’t work, even though they had it charged like 6 times. I said, oh, it’s vacuum leak, I can fix that. They were stunned, I think a little upset the otherwise fine van still had A/C and no mechanic (probably dealer) helped them. And sure enough, and $11 part fixed it (was a leaking vacuum “master” switch - the selector was mechanical