Not a pro, but I'll throw in my 2 cents.
Your compressor turns on and runs constantly? I take that to mean you see the compressor clutch engage and turn and it keeps turning....never cycles off??
If the clutch is engaging and activating the compressor and it's not ever cycling off, then I'd say the temp sensor on/in the evaporator is never seeing anything near 32F. However, if this were a low charge issue, I'd expect the receiver-dryer pressure switch to cycle the system off in order to save the compressor.
The sight glass is certainly useful, but...
Whether the system is cycling or not, you need to have a competent pro hook up some R-12 gauges.
The low pressure side could be reading low or high and/or the high pressure side could be doing the same thing. The gauge readings can tell a good tech what's going on.
The receiver-dryer sight glass is not the final word in my opinion. This test should not break the bank. The fix may be a different story.