"It's because parking brakes suck!!"
On a serious note, how do you apply it? It helps if you hold the brake pedal while you apply them. I still remember my old V code RR, which by then (1975 or so) had a 340 in, one day started it up on a cold eve to get off work. Damn thing ended up rolling back down an "almost not an incline" out into a not-very-used street and bumped the opposite curb. Still it could have hit something
