Do not rely on the sight glass on a converted R134A system as it's practically worthless for anything other than seeing if there is general flow. Newer R134A systems don't even have a sight glass. Proper charge is achieved by weight. If the system is cycling excessively, it may be overcharged and causing the high-pressure cutout switch to disengage the compressor periodically.

You really need to provide more info. Can you tell us where you are located? Climate? Ambient temps with related manifold pressures? Have you looked into the pressure/temp chart as suggested earlier?

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280