93 379 with 3406b air bubbles and pushing coolant

Its always possible an air bubble got left behind while Bleeding but it still shouldn't cause it to blow out. I'd say put a coolant pressure tester on it with the truck off and see if the pressure bleeds Off. You can also put the pressure tester on it with no pressure and start the truck and see how high the pressure gets and you can also use the pressure tester to test the cap.

 

They pulled the head and never checked for cracks? I have a friend who runs mostly 3406 in his log trucks and has a stack of cracked heads sitting next to his shop. When the coolant gets oxygenated it will no longer properly transfer heat. So after sitting overnight the air separates from the coolant, after a few hrs of runnin then you would have the same condition again. Pretty easy to find the cracks on the head most times, but maybe a small crack under an exhaust seat that was missed?

ETA: sounds like head gasket and liner fitment has already been ruled out

 

Usually pushing coolant out the cap is caused by 2 things, first being a leak that allows combustion chamber pressure into the cooling system (head gasket, liner flange, cracked head etc) If that's the case a little coolant will leak into combustion chamber when it's shut down over night. This usually causes a short term white smoke on cold start. It always causes very tiny particles of crystallized coolant in the back side of turbo. Pull the exhaust pipe shine a bright flash light in it. If they are there you won't miss them. Even if they are tiny and just a few that's your problem there should never be any. Way more accurate than testing the coolant for combustion gasses, most screw that up any way.

The he second thing is pull the front of the oil cooler off coming from the water pump. If the front of the cooler is stopped up it restricts flow and will pump enough pressure through the 2 inch bypass line out of the pump to open the cap. Front of the cooler catches all the trash in the system. I have found I ring pieces to head gasket inserts there. A few shop towels too from being left behind. Since it started after the radiator swap I'm thinking that's the best place to start. I figure it's something got left in system more than normal corrosion

 

Is the coolant changing color? Compressed air from cylinder getting into water passage. Can cause pressure in system also.