OK. Your pressure should fluctuate between 40psi and just under 20psi. The pressure must be above 20psi for the compressor to kick in.

From what you described, you are low on r134a. The pressostat is what turns the compressor on and off (or activates the clutch). With the AC engaged, both of the pipes going to the firewall should feel very cold. If the return pipe is warm, add more R134a until the pipe starts to get cold and sweat. Leave the blower on 3 or medium speed.

Forget the gauge. Most of them are wrong anyway. When adding the R134a, tilt the can sideways or even upside down after the first few minutes.

If you put in bread clips, did you use a feeler gauge to check the gap? Mine was at .046 inches, which worked fine until the compressor got too hot. The bread clips decreased the gap to .26 inch (pepperidge farm).
Did you glue in the bread clips? Did you look to see if they are still there?

The system will 'blow' the relief valve when the high pressure exceeds 550 psi. Your normal amount of R134a is 28 ounces, 26 if you live in one of the gulf states.

Klaus
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Always willing to listen, just not able to take direction.