You walk. In that case, I would walk regardless because I would ask if there were any issues with the vehicle and if a problem with the A/C wasn't disclosed, what else wasn't disclosed?

When I show up to buy a vehicle, the only thing I'm doing is taking it to a mechanic for a look over, and completing the purchase if it's all clear. That's it. The time for disclosure and negotiation is over. Not negotiating things in person (as opposed to over email where I have it in writing) has saved me countless hours over the years and made buying things much more pleasant.

Yeah, the smart move is to walk, but we're not all perfectly rational spherical car buyers. You've made all that time investment, and having cash and being adaptable is just easier.