Auto Air Conditioning & Heater Repair: recurrent loss of refrigerant, condenser coils, pressure relief valve

Question
Heavy Duty Truck: 2005 Freightliner Columbia. 2 of 3 tractors have this problem. AC will work for a short period (26 hours of operation on one unit, 60 to 80 hours of use on second) after evac. and recharge.  No leaks have been found via dye (uv/blacklite). No sign of leaks via oil traces...schrader valves have been replaced (new metal lines with valve as integral parts); new dryer; new high presssure switch (extremely high 'head' pressure was noted at one time); all condenser coils have been examined and are solid and clean; replacement of 'O' rings at the J block connections;visual inspections of compressor doesn't indicate leaks at the shaft nor at the pressure relief valve areas; no leaks can be identified via dye nor 'sniffer' tools either during nor after extented, static, high idle periods yet,,,the worst system was idled for approx 4 hours, run during approx 3 hour use; the 'head' pressure was measured and fell within normal limits; the unit stopped working after a further operating period of approx 4 hours.  Freightliner Corp. disavows any knowledge of this type problem on any units.. thanks. Dennis Sisk 303-204-9177

Answer
HI DENNIS, CHECK ON THE COMPRESSOR FOR HIGH PRESSURE DISCHARGE VALVE.DEPENDS ON WHAT COMPRESSOR IS BEING INSTALLED,THEY HAVE BEEN KNOW TO LEAK FREON IN SLOW AMOUNTS BUT LEAVE A OILLY RESIDUE.HARD TO BELIEVE WITH DYE DECTECTOR THEY CAN`T FIND ANY LEAKS..I WOULD RECOMMEND RECHECK THE EVAPORATOR.WHO IS THE MANUFACTURE FOR THE A/C?
LET ME KNOW YOUR RESULTS

GOOD LUCK