Coolant in EGR system

[Waterloo]

I would not drive that thing until you get the leak diagnosed. Sounds like the cooler is bad... But you still need to pressure test it to be sure. And that gallon of coolant is going somewhere, and I suspect it is heading right to your DPF DOC filter... Those things ain't cheap, and they don't like coolant. If that is the case, replace cooler and get rid of the mandate.