[Al Skierkiewicz]

Yuan,
The low side pressure can only remain at 60 psi if the high side pressure is greater than 60 psi. If you open the vent valve, both should return to zero. Is that what is happening?
If the high side pressure is falling after you turn off the compressor and nothing else is moving, then you have a leak in the high side. This could be a hose that was not cut flush, a leaking pressure relief valve on the compressor or a fitting not fully screwed into a component.