I take care of this machine and have for seven years since start up. It is a low pressure machine using r-123. The first five years of running 3 shaft seals were replaced , the first one was the regular type seal , 2nd and 3rd were bellows type. When each of the previous seals went bad there was no guess work : when it was leak checked the leak detectors would scream on the large setting and the halide torch would be purple. Now when the machine runs I get less than 10 purges per month but when it is shut down for 10 days or more it will fill up with air to the point where it will not run = 25 degree difference in leaving water temp vs. saturated evap temp with machine off and with chilled water flow. I have spent probably 40 hrs over the past 2 years leak checking (paying special attention to the seal) and I come up with nothing each time. I have had other mechanics perform leak checks on it just to have another set of eyes on it and they come up with the same thing ....nothing. It has got to the point now if it gets air in it I dont even leak check it , I pull the air off of the top of the evaporator and condenser , start the machine , put the liquid back in , let it purge for a couple of days and it will be back to running like a million dollars , and all I can do is hope that it is not shut down for any extended periods of time. One would think that it would be a high side leak with only 10 purges per month while running and it fills up when it is off but I cant find a leak. Any thoughts or suggestions would be greatly appreciated. Thanks.