93 379 with 3406b air bubbles and pushing coolant

hi new here posting for my father in law so bare with me

he's got a 93 379 with a 3406b cat about 80k mi ago had the engine rebuilt "in frame" had new cat packs installed they said counter bores were good head was sent out freshened up came back good everything was put back together truck ran good plenty of power. 40k after rebuild put a new radiator and cap on it and it has had air bubbles since then. they did a carbon dioxide test on coolant said it was good, bypassed the air compressor still had bubbles in coolant. they then pulled the head off and the gasket was good installed a new spacer plate gasket and put back together still had air bubbles mechanic told him to run it so he has been now another 40k later it started pushing coolant and still has bubbles , he's going to try a new radiator cap on it, he's called mechanic and was told to keep running it. looking for opinions on what it could be radiator? cracked liner? cracked head? needles to say he's not happy been trucking for over 30 years and knows something is wrong.

any help would be great thanks

 

a new radiator and radiator cap were installed 40k after rebuild. then bubbles were noticed. he was checking the coolant level and saw them day after radiator was installed. a bad cap would not put air in coolant to cause bubbles, it is now pushing coolant out of overflow so new cap is being put on to see if that is why its pushing coolant out. main question is what's causing the air bubbles in coolant?

 

thanks for the replies, the truck does not run hot its bled out, it has small air bubbles in it, he keeps the coolant at the correct level (about 2" down) its been fine for 40k (besides air bubbles) now it started to push coolant out the overflow last week , not sure if the cap is bad or if its building pressure for another reason. he's on the road now ill get in touch with him and find out what its doing and how fast its pressurizing.

like I said he's been driving over 30 years so he knows when something isn't right but the mechanic says its normal.

he move to NC last year from MA so he thought he try a local guy "that was recommended to him" to do the engine instead of the guy he been using for years up in MA who did the last rebuild and never had any issues with but he knew it was a mistake soon after the started. had a lot of problem since they did it, this is just one.

the mechanic is fixing any issues so its not a problem its the down time that's killing him.