Overflow tank

Air compressor head/gasket is prone to leakage and air pressure entering cooling system. There are other possible causes, but this is the most common failure.

 

My 2018 579 is pushing coolant when driving. Temp stays normal and have already put new head on air compressor. I’ve done a few block tests and no signs of combustion gases. I am at a loss of what else it could be.

 

How old is the head? They can fail again over time. If you replace the entire compressor with a new Paccar one it is improved design. Larger head bolts with more clamp load on the head. Next thing to test is EGR cooler, but it should fail combustion test too if it’s leaking. Are you using a gas test for use with Diesel engines. Some of the automotive fluids will always show pass on Diesel.

 

If you used the blue fluid (see video for suck and squeeze) to see if it changes color that test is not reliable. It's possible to have a bad head gasket and/or a cracked head and this test will not always show gasses in the coolant.
Watch this video.

So it looks like a head and/or head gasket, possibly block.
Like @Goodysnap said:
Lisle 75730 Combustion Leak Indicating Fluid
is there a special fluid to test for diesel combustion gasses that differs from ones for gasoline engines?
Yes, there is a special fluid known as Lisle 75730 Combustion Leak Indicating Fluid that is specifically designed to detect combustion leaks in diesel engines. This fluid changes from red to yellow when exposed to diesel combustion gases, indicating a potential leak. It is not compatible with gasoline engines and should not be used with other combustion leak testers.
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