The way to do r12 without a gauge is to watch the sight glass and charge until the bubbles disappear or just before to keep from overcharging. You only charge with the can upside down with engine off. This is done to get enough freon in to overcome the low pressure switch so the compressor will run when the engine is started. You TURN THE CAN UPRIGHT BEFORE starting the engine. You would normally do this only when the system is empty or so low that the compressor will not engage. Liquid freon will not compress so if you try this with the compressor on it will bust the case of the compressor. So, with compressor running, only charge with the can in the upright position, shaking the can to help turn the liquid in the can to a gas faster. This way does not get the exact charge in but is enough to get cold air. Or just take it to a a/c service place.

__________________
85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K