When I spend a few miles on the freeway I have to pump my brakes a couple of times for them to come up.
This doesn't happen when driving around town,
or even if it's been sitting for a while.
There is no leaks I can find anywhere
the master cylinder stays full.
Best guess - At highway speeds it's running a little hotter than around town and the M/C is heating up just enought that you are getting blow by on the seals in it. Rebuild or replace the M/C should cure it.
