I don't disagree with the advice already offered. However, I'd play it a bit tougher. Especially due to your statement that, according to you (and perhaps your inspector), "they tried to cover that issue up" of the termites. As for the air conditioning, sure it's possible they didn't know. Still....
So, I'd look at the worst case scenarios. Maybe the entire AC needs replacing, not just a repair. Maybe there's significant structural damage from the termites. I'd get multiple estimates for all know repairs, then present the highest one to the sellers and ask that the price be reduced by that number. I'd also ask for a hefty chunk of money to be placed into an escrow account for any other undetected problems. Now, they may refuse...they may negotiate...or they may do it. And the deal could fall through. But I'd say that, unless you absolutely love the house, you have enough evidence to suggest that the sellers may not be acting in complete candor. And you need to protect yourself. But definitely have a real estate professional involved in structuring your response.
Hope that helps.