I just checked the brake fluid in the small holding tank (with the dipstick) and it looks OK. I compared it to the consistency of some brake fluid you buy from the auto parts store. I’d expected it to be a little thicker (like trans or motor oil), but it wasn’t.
Yes, the problem appears only when there’s hot weather (over 80F). This evening it’s about 65F outside. I started the car, let it run a bit, drove it around for a while (1 mile), and the brake seems OK. In the same conditions of use the problem appears when it gets hot.
Thanks for the suggestion though. I thought maybe there’s air in the system, but wouldn’t air expand when it’s hot and make the brakes feel harder?