Your freon is escaping somewhere in the system.The leak must be found and repaired.Usually, it's due to worn out,brittle,or dry-rot ne0prene O-rings that are used thru-out the system in hose connections.Although,it could very well be a pin hole either in your condenser or evaporator.They are both similar to your radiator.There are leak finders sold almost any where,especially at auto parts stores.Just ask questions of those guys at these stores.Some are pretty professional.They can most likely steer you in the right direction.May get a little expensive but maybe it'll be worth it.
