Coolant leaking from reservoir overflow

If EPA 17 model- If you share your chassis number I can tell you what you have.

Test or bypass air compressor first. Single cylinder Wabco compressor has a high failure rate for blown head gaskets.

EGR cooler is possible and should be pressure tested but sounds like you ruled that out already.

From there you most likely have a cylinder sealing issue or leaking cylinder head gasket most likely caused by a fretted or dropped cylinder liner. Head gasket may fix temporarily, but it will most likely need overhaul.

 

mine had a small crack that i couldnt see till i got lucky one day and noticed it leaking there.