Evacuate the system using a vacuum pump. A vacuum pump cannot achieve a vacuum better than the Saturated Vapour Pressure of any moisture (water) or other contaminants that may be in the system or vacuum pump. This is because water or other contaminants that may have been drawn into the pump evaporates from the oil on the suction cycle and then re-condenses back into the oil during the compression cycle of the pump.
Ultimate vacuum will not be instantaneous. It is relative to pump capacity and system size. If it appears the rotary pump is not achieving high vacuum, check the following. Is the oil level correct when pumping. Are all fittings, hoses and mechanical joints tight and valves shut. If no improvement is achieved, check the vacuum pump with a known good McLeod gauge or electronic gauge by removing the pump from system, connect the vacuum gauge to a suction fitting positively sealed. Run the vacuum pump. A McLeod gauge should indicate a vacuum of between 50 and 1 micron. An electronic gauge will show approximately 250 to 20 micron after five minutes, depending on the type of pump.