now that you know where the valve is,first get a hand crank vacuum pump and attatch to vac conn on valve.see if holds vac.if not replace.if it holds leave pump attached and drive car to see if problem cured. if ok then you will find under glove box a set of vac valves that controls all of heat and ac.once you establish that good vac is coming into valve set you will need to replace valve set.they cannot be fixed. david poole e.p. dallas tx.