A/C blowing cool not cold...

Some minor bubbles are ok. If it kinda looks like foam or like a beer head flowing past the sight glass it's low on charge. The proper way to check it & charge it is by putting a set of gauges on it & checking the pressures. If you don't have gauges & are going to charge it from a can, make sure it's R-12 & hasn't been retrofitted with something else. You have to charge it through the Low pressure or Suction side of the system with cans. This is the fatter line coming out of the firewall(evaporator) & going back to the compressor. You must not charge into the Hi side(Liquid line) with a can. The Hi side would be the line running from the compressor to the front out to the condenser in front of the radiator. You don't want that one! Just charge it enough so the bubbles go away. Once you see the bubbles stop STOP Charging. Over charging the system is just as bad or worse than an under-charged system.
I hope this helps. PM me if you have any more questions. Good luck! Be cool!