Rear brake problem

[Spout]

I had to brake heavily a few miles from home on my commute today (using the seldom used back brake) and the bike felt sluggish afterwards.
The rear brake had stuck on. I tried freeing the brake manually but only loosened it enough to get me home (or had things just cooled down enough to allow the wheel to turn?)

I have now stripped the caliper and found all moving parts to be free. The piston will only return when the pressure in the brake line is released (I used the bleed nipple).

The brake peddle is 'loose' for most of its travel, and then 'knocks' against the already pressurised brake system.

Am I correct in my assumption that my 'master cylinder' is knackered?
Have I got a sticking piston in there? or is there some kind of return valve thats blocked?
Has anyone else experienced trouble like this?
Are they repairable? and how much are BMW going to sting me for a new one if they're not?

Thanks for any help

Ride safely............

Paul W

F650 Dakar

 

[littleredrooster]

Yup, not unknown for rear master cylinder to seize - mine was replaced recently under the used bike warranty wot came with the bike.