It SOUNDS as if you are using manifold gauges to monitor both sides, so that's good. If you're not, then don't proceed. In a converted system you MUST be monitoring high side pressure.
Secondly, are you sure you are charging in the low side? I hope this question isn't insulting, but it is a safety issue.
Since this system has been converted, the sight glass is not very useful any longer. Also since the system was converted before you got it, you have no way of knowing what you're dealing with. Systems can be "converted" with a death kit from Wal-Mart or they can be converted properly including complete flush, proper filter drier, correct oil, evacuation and charge.
One last thing; if everything else is okay, no excessive high side pressure, charging on the low side, etc., you can put the can in a bucket of luke warm water to get the refrigerant to boil out of the can. Don't charge with liquid by turning the can so that the hose is at the bottom. The liquid can destroy your compressor depending upon the system type.
On a converted system it is difficult to know what the high side pressure should be. I hesitate to give numbers because there is educated guessing involved. If you have over 200 PSI high side and your low side is ranging around 25 to 35 pounds, you should have reasonably cool air. Remember you lose about 20% of your cooling capacity when converting to 134, so you can't expect too much.
Your system was designed for R12, if you want a cool, long lasting system, a proper reverse conversion back to R12 is what you need depending on your climate.