There is a video here that describes a modification some people do to bypass a faulty vacuum valve. I think it also affects the a/c cutout when the engine is under load (wide open throttle or climbing a hill) which is not necessarily a good thing. That functionality is by design, as alluded to by ClassicSwede. I think the one the person in the video is messing with is the recirculation valve, but I’m not an expert on the a/c in these cars.