i have a few questions first.

you said the compressor is not on, or not coming on? if so then the low pressure reading is "invalid" as the system sort of evens out pressure when it is not on.. as the compressor isnt sucking in on the low side, so to speak.

to test this, either unplug, or jump with a paper clip (i have not tested which way yet), the switch on the AC line right by the intake. This should allow you to kick the ac compressor on and retest the pressure with it running.

do not leave it run for long, less than a minute should be enough (start car, jump /disconnect switch, put gauge on, undo switch after reading).

If the bypassed switch allows the compressor to come on, and the pressure on the low side drops then you are most likely low on r134a (or the switch is bad). You could top off the system to see if that fixes it, and recheck pressure, and try running it with the switch plugged back in.

If the pressure stays high on the low side and climbs, you either have a bad expansion valve or a bad compressor. most likely (hopefully) the expansion valve at that point. (if so i have an extra i'd let go).