Just got the d6 up and running after replacing head gasket, pre-combust. seals, and radiator core. So it runs good, but was wondering if there should be any pressure under the cap after warming up, but doesn't blow out overflow??? the pressure relief seems clean, and blew air through line, bled all the air out ( only took 12 gallons, book calls for 14.5?) seems to run at a good temp. should I be concerned or am I being over cautious? No bubbles in water while running, but still pressure even after shut down !!! just don't wan't to do this over again.

With everything right on the cooling system it should maintain a 6psi pressure. Be sure to open the vent valve on the pony motor to remove all the air.

How did the liner protrusion check out?

Liner protrusion seemed fine, checked it with an antiquated guage (not sure how accurate), by the pony vent, do you mean the bleeder between the head and the pony on the connecting pipe? or is there another bleed off plug somewhere? Also wondering if to the pressure relief is working properly, may have to test it somehow?

So what were the projection readings and was it checked with the liners clamped down? If it wasn't done right you will get to do it again.

To test the radiator pressure just connect a low pressure gauge to the overfill drain pipe with a piece of hose.

Yes, the bleeder valve on the connection pipe is what I was referring to.

What a fantastic restoration, including cab. Got a question about the fuel injection pump?? What does the decal say?? Might that be someone who rebuilt it?? Bill Walter

Pressure in the radiator after warm-up is normal, if it is not blowing water out of the overflow pipe and not making bubbles in the radiator, leave well enough alone. You won't believe the number of people who have taken cylinder heads off just because there is some pressure in the system. You warm up water or air and they will expand, the system has a pressure cap to help reduce bubbles forming and raise the boiling point a little to reduce the likelihood of damage if the needle hits the red range.

Very nice restoration, Wow! Some cooling system pressure is normal and even desirable, Caterpillar even installed water pressure gauges to be used in conjunction with the water temperature gauge in the 1960's. The water temperature and water pressure gauges had different color bands that allowed for higher temperatures to be allowed before entering the red zone when the cooling system pressure was higher. The fact that pressure remains for a time after shut-down indicates that your cooling is good condition. Please post more pictures.
Thanks,
Craig