Don't run the compressor without a full charge.

You know that the charge is gone, all of a sudden because something sprang a leak. Take care of that and it will work again.

30 psi with the compressor having not started is not good. You have a leak-out. Static pressure at 70 F should be about 70 psi. The switch needs about 45 psi to be certain to close.

Subscription required to view picture.

Have you confirmed the compressor cycles off when low side goes below 25?

Many R-12 vehicles did not have a high side switch. Those only function to cut the compressor off before a hose or something else blows out in an abnormal situation.

You need a full charge to evaluate compressor performance, so yes like you said eliminate the leak then go from there. You will probably find the system is OK once not leaking and fully charged. So, find the leak. When it leaks out that fast you can usually find it by just putting a little refrigera...

Your low side coupler is not working to open the valve in the car, so there is no way for refrigerant to flow to charge or to get a valid pressure reading. This often happens with one particular brand of cheap tools. When you start from an empty system, always charge by weight. If you charged to the...

Unplug the cycling switch while it is running-- this should make the compressor stop. If it does not, troubleshoot the electrical problem. If it does, the switch is probably bad. Make sure the switch is screwed on all the way (hand tight only though), as it needs to be all the way on to open the shr...

Whenever someone reports they see the high side a lower pressure than the low side, that almost always means that the couplers on their manifold aren't engaging the ports on the car properly.

I'm more interested in why the high side drops lower than the static pressure. That should never happen unless you're measuring wrong (couplers not fully engaged, manifold wheels opened instead of closed when measuring pressures), or there's a restriction between the compressor and the high port.

You don't have to remove serpentine belt, just turn the clutch plate by hand. When there is a blown suction reed, that cylinder would not pump, but the discharge reed will still be there to keep refrigerant from flowing backwards through it. You'll end up with a compressor that still sort of works, ...

...If the machine and operator can be trusted to put in the right amount of refrigerant. Ordinarily, seeing pressures like 90 / 15 the first thought would be "badly undercharged." Seems really weird that the high side goes up when cut off. I wonder if anything was measured properly. You ma...