Commiserations to all who have had - and those who may have experience of this problem, it is probably more common than it seems, I had exactly this with my 52reg K1200GT,

It was dealt with in the same way as the first dealer, ie tough titty, gis loadsa dosh and we'll fix it for you, went through all the possibles in this post, mostly from initiative and word of mouth. All the above are possible causes for anyone who gets this problem.
In my case I got a reasonable deal trading it in for an 08 1200GS - no servos!!!
One extra thing with mine, if I used the rear brake and the servo became permanently 'on', the disc got very hot, obviously the brake was half on. BMW tried the brake fluid replacement and filter clean and it was ok - at first, then reappeared half way from Newcastle to Edinburgh, at Alnwick 11pm on a cold night...beastly.
One way of dealing with the nightmare of - will my servos be on or off when I brake? - was to not touch the rear brake - servos always off and leave the brakes in 'emergency non-servo mode', at least it was predictable, if required in an emergency you could touch the rear brake thus initiating the servo assist. I would never condone riding the bike like this apart from in an 'oh shit I'm in the middle of nowhere' scenario.
Good fortune to all with servo assisted brake systems -
Kind Regards everyone