| Errrrmm,
I'm no expert on this, but didn't I read somewhere that people often 'adjust' the brakes using the screw that sits in the brake lever and pushes on the master cylinder actuator. I'm sure I read somewhere too that if you screw it in too far, you might think you're adjusting it for a nice firm lever feel, but you can also stop the fluid returning to the master cylinder. Wouldn't this jam the brake on after a little squeeze?
Easy to hide again as well, I'd have thought? Just tweak it back again and say 'not me guv'. | 
Jools
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