AC is a complex business especially when replacing major components, you need at least a set of gages as well as a vacuum pump to pull the system down and boil out all the moisture, you also need refrigerant as well as the correct amount of PAG oil and the correct type as it depends on the system you have. I do my own ac recharges and have for a while now. I've never had to replace any components other than a refrigerant dryer once and my tractors are from the late 70's. and will freeze your butt out on the hottest days. The y all leak down after a while, fact of life. I do the cars and trucks as well.