Itinerant Air-Cooled

Hi there, greetings from Mexico. We have been traveling through the country for a while now. The last three days I realized that the brake travel (amount of pushing the padel down) changes from little (as it always was) to a bit more every once in a while. I think that once I start the bus (1975 FI) the brake travel is more than usual. After a few minutes and several curves and brakings later the brake travel is back to normal. Than, after I stand for a while (even with motor running) the travel is more again.
Any ideas? Anything to be concerned about?
Stony

Could be the booster acting up. I'd adjust the rear brakes though.