Hi Bruno, dave Rose here... Besides you adding additional refridgerant, its important to add some oil into the system also. See if you can get a hold of a can of R12 with the oil in it already. If not, they sell oil thats mixed with enough R12 to introduce it into the low side of the compressor. When filling, dont tip the can, let the suction side slowly pull the new gas in. All leaks usually end up with some oil being released out from the system. If you do plan to service your ac system, first tighten all of your couplings where you can find them, and inspect all hoses for tell tale oil seepage. Its also a good idea to check the flange fittings on the compressor as well. Make sure you get the right oil for your R12 system, the newer PAG oil for the 134 type systems will ruin your compressor.