Replacing AC pressure switch

On a 1986 C-4 when you replace the AC pressure switch on the dryer, do you have to evacuate the freon, or can you safely replace the pressure switch without having to evac? This is the small gizmo
screwed into the AC dryer with 2 wires running into it.
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there should be a needle valve under the switch that will hold the pressure in the system. i changed the fan control switch in the high pressure line with out loosing any freon. in 88 they must of changed the location of the pressure switch.. there is no switch on my receiver/dryer..
troy

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