Series 60 EPA 07 pressurizing coolant.

Hi Guys,
I currently have a Western Star 4900 with a S60 EPA 07 egr in my shop. It originally came in at xmas time for coolant loss. We did a bottle test and found it to be pressurising the cooling system. We bypassed the compressor and it made no difference so we stripped the motor down. The head gasket didn't appear blown but after removing the liners we found #2 counterbore (which had been previously inserted) to be badly pitted and corroded. The other five counterbores hadn't been previously inserted and were only mildly worn so we machined and shimmed those and replace the corroded insert on #2. We installed 6 new cyl kits. The head was pressure tested hot and reinstalled as it was only 200,00km old. There we no traces of coolant loss form egr cooler. Anyway we got it going and its been working for 3 months (40,000km). It has now come back with coolant loss again and again failed the bottle test. We again checked for signs of the egr cooler leakage and then removed the head. Unfortunately we didn't bypass the compressor this time but have removed the head and can't visibly see any cracks or gasket leakage. The liner heights all measure up well within spec and again the cyl head has passed the pressure test hot. In hindsight I wish I did a CO2 check when engine was still together but too late now. If anyone has any ideas it would be much appreciated. This truck doesn't have air operated heater taps for the bunk or cab.
Thanks

 

No, all genuine parts and there is no sign of any damage to the fire rings on the gasket.

 

Injector tubes are ok as it passed the pressure test.
Radiator cap also ok.
Oscar the KW-due you know for sure that it was bubbling into the bottle or just blowing coolant out? Obviously the air bubbles have to come from somewhere.
Cheers

 

I already have the head off so too late for that but thanks.