A few weeks ago I was out for a ride in my 1967 Duetto. About 5 miles into the ride went to use the breaks, NOTHING! I could pump them to get a little breaking action, but that would not hold. Using the gears and the hand break I got it home without issue. In the garage I could not even pump up the breaks. In an attempt to diagnose the problem I looked for leaks. I could not find any leaks and there was no drop in the break fluid reservoir. This is an unmodified single circuit break system without boost and ATE calipers. So I thought it must be the master cylinder, what else could it be. I ordered a new MC and installed it, drained and replaced the DOT 5 break fluid in the reservoir and the same thing no breaking at all, the only resistance in the spring in the MC, not much when pressing on the peddle! After repeated pumping of the new MC, I could not find any leaks and there was no drop in the break fluid reservoir. I disassembled the old MC and it looked fine.