...And if you're not using pure R-12, bubbles mean even less. The sightglass is, by design, positioned in a location where a system filled with R-12 should have nothing but liquid flowing, if you use a different refrigerent, it will condense differently, meaning you may end up with bubbles at that same location even when full.

There isn't a proper amount of Freeze 12 to add, as the correct amount in an R-12 system is 0.

MV