Front brake failure, why?

[moto]

Curious on opinions what would cause a random failure of the front brake where the lever completely or almost completely compresses but no braking is applied.
Suspect:
- air in line
- calipers dragging causing excess heat
- crud in the master cylinder plugging orifaces
- ?????
any other

[r2nailer]

Air is always a culprit, but without knowing what you were doing prior....

There is a small seal on the plunger in the master that forces the fluid down the line. If it fails, no pressure.
http://www.ronayers.com/fiche/200_0339/fr_brake_master_cyl_-_st1100/fr_brake_master_cyl_-_st1100.cfm?man=ho&groupid=10650&parent=10630
Item 12 contains the seal
maybe or maybe not
STeve

[billxp]

I doubt very much, if its a random failure, that its air in the system. Sounds more like an internal leak in the master cylinder.

Year and mileage of the bike may help in determining what may or may not have failed, also exact conditions when the problem happens.

Keep in mind random failures are always going to be the hardest to fix.