You didn’t test the blank off value on the pump before attempting a run a used machine?!?
If the pump don’t suck, ain’t none of it gonna work…
After verifying you have a pump that will hit 1micron (or 10 if that’s all it’s good for), THEN you test the system.
I’ve had success tracking leaks down simply by spraying ethanol on the suspected location while watching the vac gauge.
see: Progress on vacuum chamber build - helium leak detection, custom flanges for HTF do I do that?