[RWH]

   
The cap for the aluminum coolant expansion tank was supposed to be a vented cap. It was not. I replaced it with a Stant, #10623 which is a vented anti-surge fuel tank cap. Outside, both caps look identical. 
Just drove back to Abilene from Lubbock. Coolant tank level seemed to respond as expected.

Thanks, all, for your input and suggestions.