The pressure switch is on the high side of the system. A 20% full system would have plenty of pressure to activate the switch and probably would pull a vacuum on the low side due to low charge.

I wouldn't be surprised to see a 1% of charge fill have enough pressure to activate the system.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician