Be careful where you buy your R-12.
Up to 80%/85% of all R-12 sold on market today is recycled R-12.
There are non-scrupulous (but inventive) recyclers going around scrap yards pulling out R-12 from old/scrapped cars and re-selling it it in nice clean bottles. You get all kind of contaminants (water, old oil, ) in that stuff.
Not very good for a freshly rebuilt system
JackD