so apperently every single time i have a passenger, my rear brakes lose all pressure. not immediatly but after a few miles of stop and go traffic. when i ride myself the brakes rebuild pressure by themselves. i took apart the master cylinder and didnt find a ripped cup like others have had. i brought it to an indy and them check it out but it was working that day so they said it was fine. i took the caliper apart also and cleaned and lubed it up. bled lines and worked like they are supposed to, then ride with a passenger and they lose pressure again. its never happened when im riding solo. the easy fix is to not ride a passenger, but seriously what the hell is going on. too much weight, me and my ol lady arent that fat. lol
What fluid are you running DOT 4 or DOT 5? Might be that the extra braking effort is boiling the fluid - and that will cause fade. DOT 5 has a higher boiling point than DOT 4 and mixing DOT 4 and 5 will cause all kinds of issues.
i had a master cylinder once that i had to tap the brake real fast before i pushed it, if i didn't the petal would go all the way down with no braking power. disassembled it several times, found nothing wrong with it put a kit in and still never fixed it. not much fun when your running rear brake only. I replace/upgraded to a better one.
1979 Ironhead Chopper aka Home Wrecker
1995 1200 on a modified 1980 frame rigid mount swingarm
2008 Nightster GF's bike
Why buy parts, when you can pick up some tools and make something way cooler?