I'd say you are still low on gas. A/C systems can behave kinda strange when low on gas. System should take more R12 without any trouble. You are charging with the engine running and the A/C running at max speed, right? Should be able to gas charge (car upright ) into the low presure side. Set the can in a pan of warm water to help encourage the freon out of the can and into your car. I usually shoot for 35 degrees F. on the low presure side. Forget exactly what presure that is with R12, think maybe around 35psi. Do this at around 2000 rpm. Then stuff a thermometer in the air vent and go for a ride. See what outlet temp your get and how much of a split between cabin temp and outlet temp. On a moderate day, I like to have a 20 degree split.