Do I have to extract the refrigerant from my AC to change out the pressure sensor? I have a 1999 Chevy Blazer.

i didnt have to on my 95-s10. if your speaking of the one mounted on the accumuator, its schrader valved (mine is anyway).

Which switch High pressure relief or low pressure cut-off. The cut-off is screwed to the accumlator and it has a shrader valve in it. No Need to suck out the gas on that one!