Best guess - At highway speeds it's running a little hotter than around town and the M/C is heating up just enough that you are getting blow by on the seals in it. Rebuild or replace the M/C should cure it.
Disagree w/ heat at highway speeds as my original brake '49 daily driver summer trip 1k miles is indifferent to heat. I do agree with checking the status of the MC, I had mine resleeved in brass.
Oh, and if you're running bias ply, drive alone and not with any loved ones and of course NOT WITHOUT SEATBELTS. Bias plys on that truck were meant for dirt roads at 30 mph, not 65 in modern traffic.
~ Cosmo
1949 Chevy Half Ton
Rocinante, like Don Quixote, he is awkward, past his prime, and engaged in a task beyond his capacities.
"...my good horse Rocinante, mine eternal and inseparable companion in all my journeys and courses." ...Don Quixote, Cervantes
"If you come to a fork in the road, take it."...Yogi Berra
"Rudeness is the weak man's imitation of strength." ...Eric Hoffer
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