Originally Posted by
Peter_1
Some thoughts because the original poster is from Europe and some seems not to be aware about international regulations.
Cast irons pieces have to pressurized at 1.3 x max pressure.
LP and HP may have different test pressures, related to their max allowable pressure.
You may not add a synthetic refrigerant to trace a/the leak. He, Argon,... is allowed.
If you add to the OFN some He which penetrates better then every other gas, how will you then detect this leaking helium? Handheld He-sniffers are not available, only heavy and very expensive mass spectrometers can do the job.
Cool Runnings gave the same answer as I should gave you. Pressure test showed you the leak or bursted a weakening place in the bushing.