Problems with front brake

I've got a problem with my front brake. When I hold my brake lever in for a while it gets softer and the lever comes back to the bar. Then when I leave it, it goes back to normal again. It isn't really noticeable when I'm riding unless I'm stopped on a hill or riding down hill for a while. The brakes have been bled twice and I can't see any fluid leaks anywhere. Have any of you had this problem or any ideas how to fix it? Cheers

the master needs to be rebuilt or replaced it is losing pressure

Ok, I'll get on to it, cheers. Yes it's the 990. I have been changing the fluid every year. Do you know of any good aftermarket master cylinders that will fit? I'd rather not spend over £200.