If your putting in a pressure switch you will need a special adapter. to run the gauge to, Its metric. A good way that works to do this and is cheap is the use the old sending unit and then drill and tap it to take the gauge .Works great. We had the same problem on are GXP build but after we charge the sender with a new A/C Delco unit, and it was fine. We first checked it with a gauge to make sure we had pressure Hope this helps you.
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