2012 Kenworth 660 Cummins ISX pushing out coolant

I have a 2012 Kenworth 660 with a 2250 engine it it. The truck has 1 million miles on it and to this point has been very reliable. I did an oil change recently and had the crankcase filter changed and the tech informed me that the filter in the crankcase had moisture inside. About a week later I got a check engine light and took the truck to my mechanic. Mechanic said that engine was building too much pressure. He replaced the crankcase sensor, crankcase filter and the truck ran good for about two weeks. At this point, I started to lose coolant. I called my mechanic and he told me to let the engine cool down and start the truck with the radiator cap off. The radiator pushed all of the coolant out even though the cap was off and the engine was cold. Coolant was pushed out within 30 seconds. I was in the middle of nowhere when this happened. My mechanic told me that most likely it was the air compressor because the truck was running fine. I drove the truck to his shop and added coolant every 70 miles or so. The temperature ran at 180 degrees and the oil temp was normal and the mpg was average. The truck ran as if nothing was wrong. My mechanic checked the turbo first. He then dropped the coolant and isolated the air compressor but this did not relieve the pressure in the coolant system. The truck is running smooth and temperature and mpg are normal. Apart from pushing out the coolant, you would not know anything was wrong. There are no codes and there is no coolant in the oil and no oil in the coolant. I NEED HELP AND AM THANKING YOU IN ADVANCE FOR ANY SUGGESTIONS. My mechanic wants to remove the injectors and replace the six injector sleeves.

 

Thanks for the advice. 1,020,000 miles and has served well. Engine has never been opened, not even for an overhead. It burns very little oil and at this point I am weighing my options but I want to make an informed decision. I have been reading the posts on here as well as watching the videos on Rauze's web site. Is my mechanic going in the wrong direction by writing an estimate for removing the injectors and replacing the injector sleeves. The truck is a 660 studio and I definitely dont want to get rid of it. It is currently at a paid parking space so I have time to weigh my options.while seeking advice. I am currently in the process of buying another truck while this one is down.

 

I am not a mechanic so I appreciate any advice. My biggest asset at this point is time as I dont have to rush to get the work done while buying another truck; This 2250 400 hp gave me over 1 million and I am currently buying another cummins with the same engine.