a/c low side goes into vacuum?

Good morning.

The low side should be 25-30 rock solid, no variation.

It looks like you need the expansion valve. That is what controls the low side pressure. When it fails, it will put the low side into a vacuum.

Roy

You need to recover the Freon, remove the low side manifold nut at the firewall and then the two bolts securing the valve to the evaporator.

You will need to replace the receiver/dryer as well since you did not at the prior repair.

Roy

A/C went out suddenly. Low pressure A/C line read over 100 psi, so didn't recharge. High line read okay. Compressor clutch won't engage, hesitant on jumping clutch. Bled low line until down to 40 psi last night, only air came out, no Freon (at least until when I stopped). Came home today and psi is over 100 again, low side. Fuse is fine, have read relay is inside computer. What else can I check myself? Was thinking pressure sensor, or maybe blockage? DIY here, moderate car knowledge.

A blockage is normally only when you have low pressure on the low side and high on the high side. When you are taking these readings, was the compressor running?

If not, we can assume this is just a normalized system. When the compressor is not running and the system is static, the high and low side will actually equalize.

We need to start with getting both actual pressures when the engine is running and A/C on. Take note if these pressures are with the compressor running. If the low was in this condition then what was the high side pressure because you said it was normal, so what is the actual number?

https://www.2carpros.com/articles/car-air-conditioner-not-working-or-is-weak