I had that happen on my vett. It took me forever to figure it out... do you have any pulsing when you apply the brakes on the highway? The problem I had with my Vett was the calipers are spring loaded and the slighted out of line that the rotors have will suck air into the calipers.
True story, I still do not know how it happens, but I did get it fixed.
i'm still getting air in my lines when i drive over 60 for a mile or so (ex. interstate highway) but i can drive around town for weeks with it being fine.
I can't find a leak in the lines and I've checked the vacuum line out of the master and everything seems fine.
I am about to take it to a professional and have them figure it out, but I don't want to pay to have them fix something that doesn't need fixing...
Fierst are the brakes adjusted correctly? They usuallly aren't after some time especially drum brakes. The self adjuster threads never get lubed. Are you losing brake fluid out of the MC? Check the inside of your tires. You may have a leaking wheel cylinder. Or the MC itself is shot. If you are not losing fluid but have no brakes at higher speeds when you hit the brakes, the rubbber lines on your brake system could be expanding which will increase the area your brake fluid needs to fill, making your brake pedal go to the floor.
Your are either losing fluid, increasing the area of youur fluid space or your MC is shot.
Do you know what was good about the good old days? I wasn't good, And I wasn't old!
Senators and Congressman should wear uniforms like NASCAR drivers so we could identify their corporate sponsors....
If you have power brakes just for the heck of it pull the master and see if you have a leak. Years ago a guy brought me a car he couldnt keep brake fluid in everything was new no sigh of a leak. Long story short the booster had a hole in it and sucked the fluid out. I know you are not loosing fluid but it cant hurt.