The pressures you mentioned that "shot up" to 75 and then 100psi is a concern. I really need to know if that was the high or low side pressures. If it was the low side, something must be plugged for it to climb that high or the compressor is going bad.
As far as the 75 with the compressor not engaged, that is considered static pressure. However, usually you need to wait a bit for it to equalize. If the static pressure is 75 PSI and the ambient temperature is around 75 degrees F, that is normal.
What I would really need from you is what the high pressure and low-pressure readings were then the system is operational. In addition, the ambient temperature makes a difference, so that is needed as well.
As far as the compressor being bad, it is very possible. The idea that when it was running it showed 100 PSI, which is extremely high for the low pressure. Here is what to keep in mind. If the low-pressure side is high and the high-pressure side is low, suspect the compressor is failing. If both sides are high, it's likely overcharged.
What I suggest is to start from the beginning. Evacuate (vacuum) and recharge the system using A/C manifold gauges. Here is a link you may find helpful. It explains the procedure. Note, make sure not to add too much oil.
If you determine the compressor is bad, here is a link that shows in general how one is replaced:
If you look at the attachments below, they provide the directions specific to your vehicle for compressor replacement and adjusting lubricant for compressor replacement.
I hope this helps. Let me know if you have other questions.
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Monday, July 26th, 2021 AT 9:24 PM