Oh man what a pain. I had that happen to me on a commercial kitchen walk in fridge. You might not like the news but...
That sight glass/line needs replaced, a seal has gave up letting refrigerant and oil escape.
-The refrigerant needs to be recovered (EPA rules)
- Repair/ replace defective part
-With a set of AC gauges, Pressure test system with nitrogen to check for leaks
- If no leaks present than pull a vacuum on system to rid of any air and moisture( the enemy of ac systems)
- Re charge system correctly with refrigerant.