My high pressure plug blew because of condenser fan failure. A binary switch inline with the compressor clutch relay will prevent the compressor from engaging if there is either too much or not enough pressure in the system. The receiver driers have a fitting on top to accommodate the switch or the sacrificial plug.
And like Larry said, the R12 is easy to get. Not cheap, just easy. I bought some on Ebay on a Saturday morning and it was hanging on my mailbox Monday.
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