Brakes question

[Jacko]

Hi
My front brake has gone soft.
It has more travel than usual but even stranger it loses all
pressure and the lever comes back to the bar and no braking at all.
squeeze again and back to soft pressure but brakes work.


Going to have a look at weekend but any ideas on where the likely fault is.
ie: master cylinder, calipers, lines,  everywhere??


its a 98 600 with 68K on clock and as far as I know original lines and calipers.


Thanks