Your assumption of what the pressure switch does is correct. While the switch itself can be bad, most often it's just doing it's job by shutting off the compressor when pressure is low to prevent compressor burn out due to low circulating refrigerant (and therefore oil). With 134a you just charge it up (with gauges )to see if it engages the compressor at the proper pressure. If it does, you then "seek the leak" that led to the low pressure situation. If charge pressure is correct and it still doesn't engage the compressor.....usually bad switch. Evacuate refrigerant and replace switch.
The sight glass "piece" is the receiver/dryer. Sight glass charging is of no use with a 134a conversion, so use gauges for sure.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)