It sounds like the vet is draining fluid from the chest cavity; that is, from around the lungs rather than from in the lungs. It would be worthwhile to send the fluid out to be analyzed. From your posts, I gather that the cat has had intermittent respiratory problems, all of a similar nature, most of its life. If the fluid looks milky, it may have a chylothorax, which is chyle, or lymph, escaping from the ductus arteriosus on its way to the heart. I doubt that a chronic pleuritis could last years or not cause a constant pain. Get the fluid analyzed.