Not piston related but brakes of the Caparo variety on a 2009 4/4 Sport. At around 20,000mls the car would tend to want to jump into the hedgerow under braking.
I stripped the calipers down armed with new pads and discovered that one of the offside pads had delaminated from the steel backing plate.
A lot simpler solution than I was anticipating.
Not sure that you're the first
see here. 
