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As with most vehicles (although some folk don't realise) the handbrake serves as an emergency brake in the event of total failure of the service brake (usually split into service and secondary systems).
In the days before dual circuit systems, the handbrake (usually mechanical) was the 'last resort'. Although parking brakes are now typically spring operated, the control valve has to be 'progressive' such that the mechanicals don't break if applied when the vehicle is in motion.
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