Having thought on it for a few days, I can only assume that there has been (and still is) some water or other impurity in the brake lines. It's the only thing I've ever seen cause this binding over time as the brakes (and therefore the fluid) warm up.
I've changed the fluid twice now so it's time to completely strip the system, check all the pipes/joints and clean with some denatured alcohol or similar. Then get it back together without letting so much as a sniff of condensation back in.
Guess that as a precaution, I should change the seals, too.
Those long winter evenings will just fly past...