I think both Conor and Jake are missing a point. Taking your foot off the brake, without the handbrake being applied… 
Liquid tankers, whenever stopped (but especially behind stopped vehicles, in front of pedestrian crossings etc), always handbrake on. Always.
For what it’s worth, I’ve driven milk tankers for 10 years, single pot with and without baffle, and twin pots. Ex-farm and reload.