I believe it also acts on a seperate braking system - drum I think - located within the wheel hubs. I was informed by OPC that the pads for this can oose resin if hot, and stick. The next time you drive away (next morning say), you get a disconcerting 'break away' from each corner as each stuck brake comes loose again. I think this
may happen even if you don't apply the hand brake, and is normal.