Is there a check valve behind the A/C pressure cycling switch or do I have to drain down the system to change it?
the low preesure cycle switch does have a check valve under it , yes
Unsure of your question but yes. There are valves on the ac system used to charge and evacuate it. Letting the refridgerent into the atmosphere is illegal, stiff fine from the EPA.
Re-read the question. There is no way to change a part of the ac system without removing the refridgerent.
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