General theory Bob is that air (and moisture/condensation) gets in through the seals, especially at the rear wheel cylinders. This is why the rear cylinders get so rusty around the edges.
And the fluid from the rear system is always a lot more contaminated and black than from the front.
Very true I had one 4/4 that failed at the rear claipers and have just replaced them on Belle at 10 years and 36k as they were starting to fail. Not helped I suspect by a failed oil seal on N/S axle.
