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The newer cars(96 and up) use r-134a freon) If you were to recharge the system you would have to discharge or evacuate the system first. Since moisture is very bad for an a/c system unless you have tools to evacuate and charge I would recommend taking it to a shop. You might have a leak or just be low. Automotive a/c systems have what they call calibrated leaks, with high pressure and moving parts with seals(compresssor) they leak. Most vehicles recommend service every 4 years. Machines and gauges have hoses that connect to service ports(high and lower side) That is how you evacuate and recharge. If you are going to just release the freon and add I can tell you the a/c performance wil stink