Instruction to release park brake when unloading

[1955LR]

Can anyone explain the reasoning behind this notice on the side of this wagon. Thanks

 

[Snow1964]

Normally it is because hand brake is manual, and mounted in such a way that as a vehicle unloads it rises on the suspension and the link from brake lever gets tighter so becomes harder to undo, or effectively becomes jammed on as too tight to move.

 

[Dunfanaghy Rd]

Back in the Stone Age,when I was on PW, we used to get Grampus wagons of S&C timbers to unload at our stores. Lever hand brake, pinned down. Wagon unloaded. Lever unpinned (with difficulty) and wagon jumped about 3 inches into the air as the grip on the wheels was lost and the springs reset.
Pat

 

[1955LR]

Thanks both. I wondered if it was a suspension relaxing issue, wasn't sure if it would get tighter or perhaps release and give the wrong impression.