Does it have sections?  If so, it could be the master on/off not completely closing.  My money will be on bad O-rings in the cooler.  Lots easier to find leaks with air.  Put an air fitting in a bleeder port and you may be able to hear the air.  It's coming out of both coolers (all 4 vapor hoses), is why I mentioned the on/off valve.  I assume it's worked in the past?