I just had my master cylinder fail on the KZ550 that I bought new in 1980. In 1983 I added a second front caliper and braided Russell lines; I flushed and filled with DOT 5 silicone. So yeah it eventually failed but (1) it took 33 years and (2) it didnt hurt the paint on my nice original tank when it emptied itself onto it. Oh and (3) I was easily pulling fronties on it the whole time, running two calipers on a one caliper MC.
I'm draining everything right now, but I think I'll put the other half of that bottle of silicone DOT 5 back in.