Thanks for the suggestion. My first thought was something along those lines, but there is some information that suggests to me that this is probably not the case.
When the bike was delivered it was to a friends business, 5 miles from my home. I rode the bike, damaged, to my home and had no problems with the brakes. The distance was double to my ride yesterday when the brake problem showed up. During the rebuild the braking system was not disturbed, except for the calipers being unbolted from the forks, the master cylinder was not removed or worked on in any way.
The fact that when the system has cooled down it needs to be given one full brake lever pump to get the lever solid again leads me to think that it is probably a fluid issue, although not returning to the master cylinder/reservoir, or returning too slowly, is also a strong possibility, although with nothing changed its difficult to understand why.
I appreciate any further thoughts.