Detroit s60 14l coolant overflow

I got 09 westernstar with ddec vi Detroit s60 14l.lately it started pushing coolant out of the reservoir.loosing about gallon a day.so far no coolant in oil.no visible leaks.EGR cooler is welded up so no white smoke.truck is not overheating and runs fine.so far i did the combustion leak test with leak tester kit.the fluid turned green which indicates there’s exhaust gases in an engine’s cooling system.but i also did the bubble test but found no bubbles at all.i know the EGR cooler is cracked internally that’s why i welded up before.now even the EGR being non functional can the exhaust still leak into EGR cooler and cause pressure build up in the cooling system??i want to rule out the EGR cooler before i tear down the engine for possibly bad head gasket.any suggestions or other tests i can do to rule out that its not head gasket??

 

It seems to do it more while under load.my last run was out to west coast and it would throw low coolant light while pulling hills.i guess it time to tare it down and see what we got going on