Coolant abnormally passing to overflow reservoir

2010 Peterbilt 389

For the past week while driving, the coolant in the main reservoir keeps passing over to the overflow reservoir in an unordinary fashion. We started the past few days just under the maximum level. While driving 400-500 miles, truck will randomly set off the low coolant warning on the dash; and the overflow tank will contain significantly more coolant.

Current coolant level: halfway between min/max. Right now I am inside with hood open and idling, with rpms set on cruise @ 2000. Doing this, the main reservoir level drops noticeably and immediately. When I let off the cruise and rpms back to normal idle, the reservoir fills back over the minimum in less than one minute.

No leaks

Any ideas what this issue could be?

 

Oil is fine, nothing abnormal coming out of stacks. Taking to the shop tomorrow with the head in our crosshairs. We just changed the EGR Friday. Will take a look at the air compressor, I appreciate the replies.

 

ISX 485hp

 

We changed the EGR cooler Friday, correct. Ran it Saturday and this morning, all was well. Resumed aforementioned symptoms this afternoon.