I had one of my beginner tech's respond to a service call on a large R12 system that cools an industrial control pulpit. One of the gage lines blew off the high side and 'some' of the charge was lost to the point it was no longer cooling well. He thought that MP39 was a drop in for R12 and added about 10 lbs into a system that contains about 100 lbs R12. It is working but is not cooling well and lots of bubbles in sightglass (may still be undercharged). TXV and receiver in system so subcooling not a good indication of charge. Any ideas other than recover and refill?