C-15 losing coolant from overflow

No air bubbles in water bottle test, changed cap from a ten lbs. to 15lbs. problem persist. Any ideas would be greatly appreciated.

 

It will lose roughly 2 gals. rather quickly. Then stedily drip drip until the resevoir is empty. I can't get it to do any of this while parked, low idle, high idle, no matter won't spit a drop. Ran it at a high idle in driveway for about 90 mins. nothing. drove it 5 miles down the road steady stream out of the overflow. By the time I got back to the house had stopped and only the drip drip drip. the dripping will lose about 1-11/2 gals. in about a thousand miles. To give a little more detail is c-15 w/ 6nz prefix about 370,000 miles on rebuild at caterpillar. Otherwise seems to run great fuel mileage is typical avg. 6

 

Thanks Les, I'm hoping so. I've read as much as possible in the forums. I guess the no bubbles is what is throwing me. Are bubbles always present when gasket or head problem?