Mike and I measure the resistance between x-ray anode and ground using the megohm meter, it was about 5.8M Ohm as applied 1kV test voltage. It's definite that the front or back bushing was not good insulated. So I took out the anode bar to check the front bushing. I found the epoxy insulator looked very dirty, there was oil track on the surface. I think the silicon oil might leak inside the x-ray gun when we installed the anode bar. We cleaned and repolished the bushing, it looked more cleaning(shown as the top picture).
I measured the resistance, which was 50G Ohms. But the measured resistance became 5.8M Ohms after we inserted the anode bar into the x-ray gun. I will have to pull out the x-ray gun to check the back bushing next week.