i had the problem described with the m/c , but i hadnt messed with tire or caliper or brakes at all for about 7 months maybe. just went out while on a ride. no resistence when pushing brake. im about to disassemble now.
every rubbermount I see with this problem was because of a tire change or removing the shocks letting the belt slam down on the linkage right where it turns to go into the master cyl.
It shows up while changing a tire, but there is no connection. I recently spoke to someone who is an engineer for that MC. What seems to happen is that the primary cup gets torn in the center and it manages to still work as long as the MC is not disturbed much because it is backed up with that flat metal plunger which helps it seal even tho broken. Then they are a bit of a bitch to bleed, but with a broken cup they won't bleed anyway.
Those of you who have done the job will note that the new cup has a tit on it that goes into the center hole of the washer that is welded to the end of that spring. Well when the parts are new the problem doesn't seem evident, but if you take the broken cup and the spring/washer thing and hold them together on the bench you will see that the washer can slide sideways quite a bit within the center of the primary cup. I think that the tapered spring tries to cock some every time the pedal is pressed causing it to work against that tit until it breaks the tit off and leaves a split across the cup. I think the better cure would be to put a larger washer on the end of the spring so that it will not keep fighting the tit for centering. I sent a detailed explanation of this to the person I spoke too, but did not get a reply yet.
Bleeding is the other problem, and the big loop in the brake line is a possible cause for this (trap for air), and also some have said that the return hole in the MC is too small (I agree), but enlarging it is risky unless you can make sure that there is no burr left to hurt the new cup.
A Biker is not just any guy who rides... that's a motorist.
A guy that gets up and rides again after every crash... that's a Biker.