back in the days when i had a company car i parked the nearly new mk5 cortina behind a landrover , went in the dealers office for a meeting , came back 3 hours later to find the cortina had moved forward a foot, bemper resting against the landrovers quarter bumpers with the spotlights between them. ivenever trusted handbrakes since
this morning i went out to pick up some rockery stone.... reversed the car and trailer up the sloping drive, applied the handbrake firmly, car and trailer stationary on slope pointing at the open gate and road. so selected first gear.
hour later with about a ton of stone in the trailer i hops, in foot on clutch to start car.... it starts to roll forward with handbrake still on. was well within the cars 2 ton towing limit so handbrake should have held the weight on its own.
all handbrakes are held on by a cable under tension , im convinced they stretch a bit under load and slowly release a tiny bit of tension over an hour or so. not normally enough to notice but just enough when youre on a bit of a slope with weight on.