So my truck (1996) was low on R134a - it has a slow leak. Just started getting hot from a charge up about 2 -3 years ago so I have put off fixing it. I started refilling it today and was aiming for 28-35 PSI on the low side per the manual. I was on my way up to 35 around 32 PSI (I was stopping the fill to check intermittently) when all the sudden the AC compressor locked up and the belt spun around. During the fill process, it never clicked off once. I evacuated enough that the AC ran normally (feels good and no lockup) and at 1500RPM I'm seeing upper 20's for PSI on the low side. Not quite where it should be but its blowing cold so I'm not going to fuss with it. The question I have is shouldn't the compressor be cycling on and off? And shouldn't the overpressure switch be shutting the clutch off when it gets too full? I kept adding because the compressor never cycled. It is old and probably original so I'm considering that I may just need to replace it, but is that overpressure switch internal to the compressor? Also am I wrong in my thinking and approach to troubleshooting?