Having messed around with 'models' (Post-It pads and rulers etc ) I think that sweep wouldn't produce any more difference in castor than is produced without the sweep, i.e. the 'dive' effect of the chassis under braking.

Skiddins

Although I do not believe it is this so-called "reactive caster" that gives sweep it's trait, I am having a difficult time seeing how sweep alters caster as the suspension goes through it's motion. To alter caster within the confines of the suspension motion, the angle (or relative heights of both the leading and trailing ends of the outboard pin) would need to change as the suspension moves about. In most of our car's designs, the caster block is mounted at a fixed angle to the a-arm, so outside of a-arm flex (twisting, as softer arms do) the caster angle does not really change as the suspension is compressed, sweep or not. Now pan cars have something different going on because their kingpin angles are not affixed to their suspension motions, which gives them the ability to have true "active caster".

Very good explanations, Cristian and Lessen.