If your Galant was manufactured after 1995, recharging the system with R134 refrigerant is possible as long as all of the components are working properly. Models made in or before 1995 may use R12 refrigerant and will need to be retrofitted to accept R134 refrigerant before you can recharge the system.
Look at the air conditioning lines for signs of damage. Leak detection kits are available to help detect leaks and seal small ones. If you notice any leaks or damaged areas, you should replace or repair them before you recharge the system. Keep in mind that it is illegal to discharge refrigerant into the air. This includes refrigerant that leaks from a recharged system that you know leaks.
Find the Low Pressure Side port valve. This port is between the compressor and the firewall on the driver's side. It has a blue cap marked L or LOW. Take the cap off and save it. Look at the Low Pressure Side port connector to determine if the system is compatible with R134 refrigerant. If you see quick connect valves, it is R134 compatible and you can continue with the recharge. If you see screw on valves, the system takes R12 and you need to retrofit it to accept R134 because R12 and R134 are incompatible. Mixing the two may cause serious damage to the system.
Start the engine and the air conditioner. Make sure it is set on the highest fan speed and lowest temperature to get an accurate pressure reading. Check the pressure on Low Pressure Side of the system using an automotive air conditioner pressure gauge.
Attach R134 refrigerant to the Low Pressure Side using the hose or trigger mechanism included in the kit. Make sure you follow the directions on the kit. These will vary with different kits. Keep in mind that it may take more than one can to fully recharge the system. Most Mitsubishi Galant models can hold a maximum of 27.4 oz. of refrigerant. Make sure that you check the pressure often so you can be sure to keep the pressure within the safe or normal range.
Close the valve and remove the refill kit or can from the port valve. Follow the manufacturer's directions when you do this. Make sure that you replace the cap on the Low Pressure Side port valve.
Leave the engine running and the air conditioner on for at least 15 minutes. This gives the refrigerant a chance to circulate through the whole system.
In some areas you need a license to purchase all types of refrigerant, including R134. If you live in one of these areas, take your Galant to a mechanic or service garage to have the system recharged. Wear safety goggles and gloves when you work on the air conditioning system of your Galant.
Don't try to service the High Pressure Side or overfill the system. The air conditioning system is under extreme pressure and may explode causing injury or death.