I have a factory installed unit in 1982. Due to leakage the unit unit did not work in 2016. First Check the unit for the type of Freon used there will be a label on the compressor assembly. If it’s old Freon R 12 was used. A mineral based oil. You need to pull the unit to add a fill port on the compress. Then have the proper fill of R 12 around 3.5 oz to the Unit after a complete vacuum ie -29mm. I suggest a 24 hr vacuum check ie no change in readings . Note any work with R12 is controlled by the EPA and done by a licensed person also R12 is no longer made. If you can fine a old falcon air horn can from the 1980’s or on eBay good luck. The replacement for R12 available use flammable gases not allowed in the US and not on a boat. I Had a qualified person doing the above with success, only to find out the tubing quick disconnect leak out the Freon after several months. Parker Sell replacement quick filling but your back to were you started. My solution was to bit the bullet and replace the unit. Defender Marine has a sale in the spring. Oh! The cost of a cold can of beer.