It is widely assumed that trailer spring brakes are applied simply by disconnecting the red (emergency) line and that the park button does nothing more than this.

Not only is this assumption about the so-called fail safe nature of trailer brakes technically inaccurate, it could prove positively dangerous at the time a trailer is re-coupled. 

The risks involved in failing to apply park brakes properly on motive units and trailers have been evident for some time, most starkly illustrated by fatal accidents.*

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The Pownall Auto-Set Trailer Park Valve* can be manually operated at any time and does not prevent or relieve drivers of their duty to follow safe working practices. However, should the driver 'forget', the Pownall Safety Park Valve automatically sets acting as a safety backup.

Marks & Spencer trailer built by Gray & Adams - IRTE '98

Close up of trailer controls.  Pownall valve lower right.