Manifold gauges help you understand what the sight glasses are showing you. Put your gauges on the unit. The low side will go on the larger line. This will be the blue hose. The high side will go on the smaller line. The high side gauge is the red line. They both screw on to the Schrader valves on the appropriate lines.
Refrigerant Charge is Low
When bubbles are present in the sight glass, it normally indicates a low refrigerant level. If the low pressure gauge indicates a low pressure according to the ambient temperature of the conditioned space and the head pressure is low, there is probably a leak that needs to be fixed. If it's a case of tightening a fitting or something of that nature, make the repair and charge the unit until the sight glass is clear. If the leak requires soldering, the refrigerant will have to be recovered and the system evacuated and recharged until the sight glass is clear.
Air is Present in the System
Seeing bubbles in the sight glass can also indicate non-condensable gases in the system, such as air, which can be introduced to the system by charging the system with refrigerant and neglecting to purge the gauge hoses to eliminate the air. If this is the case, the high side gage on the manifold indicates a high head pressure, while the low pressure side might run low due to moisture freezing in the restriction device. In many units, the sight glass has a dot that changes color if moisture is present. This also requires recovering the refrigerant, evacuating the system and recharging the system. The liquid line drier should also be changed.