Cat c15 Sdp air locking

That elbow will be double o-ringed. It almost looks like it was pushed out by excessive pressure unless the mechanic didn't push it all the way in.

 

That cast manifold definitely doesn't look right. Its too shiny on the end, which leads me to believe it should be pushed in another 1/4" or so.

 

bubbles in header tank and excessive coolant pressure would indicate the system is pressurizing.

get a water bottle and run over flow hose into it with bottle 1/2 full. cable tie it infront of your windscreen so you can see it when you drive.

go up a hill with some weight on in order to keep the engine under load for a period of time. if that overflow hose blows bubbles you have either cracked flange or liners have sunk and head gasket is bypassing

 

well it would seem the liners have either sunk or its cracked a flange as you have previously said.

time to pop the lid off and have a look

 

both would cause compression gasses in the coolant.

have you tested the coolant showing no signs of combustion gasses or done the water bottle test even at idle?

sometimes I find in the early stages of failure they will only blow bubbles with a low cap like 8 or 10psi.