I've always heard the sight glass is not useful for 134a systems. Newer cars have had the sight glass removed from the receiver/dryer - my 210 doesn't have one.

Charging until the sight glass is clear on a 134a system almost always means the system is overcharged, and overcharging is one of the worst things for an a/c system.

One explanation for the cloudiness/bubbles in the sight glass is the (usually) PAG oil inside the a/c system is less miscible in 134a than the old mineral oil/R12 systems. Hence what you are seeing is an emulsion of liguid refrigerant and oil droplets moving through the system. Adding more 134a increases the high side pressure and temperature until the PAG oil finally mixes completely - that's when the sight glass clears.