[Beav]

Normally one assumes that R-12 will yield 1 degree per 1 psi on the low side. R-134 is close enough to consider the same. Soooo..... most systems will operate best at approx 31-34 psi on the low side when full and spinning at ~2,000 rpm. Give it a few minutes to somewhat cool the interior as elevated interior temps will affect the reading. Lower pressures will usually result in compressor clutch cycling due to low refrigerant charge levels.

Watch the sight glass, if the interior is below ~80 degrees, the low pressure is 30-35 psi and you still show bubbles (other than a few random ones) you may have air/moisture in the system which will require an evacuate and recharge. Evacuating not only means draining the refrigerant but also placing the system under deep vacuum for a period of time in order to cause air and moisture to boil, in order to be evacuated.

R-134 is very prone to poor cooling due to incorrect charge levels. A few ounces one way or the other will cause poor performance. Also those that succumb to Moore's Law will eventually learn that over-charging the oil in the system will cause various maladies, including the fact that the more oil in the system leaves that much less room for refrigerant.

Same goes for those that think installing leak sealer is a good idea. It works by sealing when it comes into contact with air. Hmmm... now if they had a system leak and didn't evacuate the system properly before installing the sealer, the system will be full of air... Next, after finding it doesn't work at all, they take it to a repair shop. They are too embarassed to tell anyone that they tried to fix it and couldn't. So, here goes the tech with his $4000 a/c recycler and (especially if it's R-12) expensive cylinder of refrigerant. First move, hook up and check pressures, allowing uncured sealer into his machine. Then a quick evac and recharge and he has a contaminated cylinder of refrigerant. Nice. Now the customer is responsible for maybe $2000 or more in damage to his own car plus another $4000 - $5000 to the repair shop's equipment. I just had to throw this in because I've seen it happen to many shops when I was selling tools. It's never pretty.