2003 HDi Handbrake - rog_the_dodge
My 2003 406 HDI Estate has always had a useless handbrake, but I have found an easy cure.

Since the handbrake uses a drum within the rear disc, it never gets any wear on the linings, and in fact I don't think mine had ever got properly bedded in.

Simply run (preferably down a steepish hill or two to avoid fuel wastage) with the handbrake firmly on,to bed in the brake linings properly, and the handbrake will subsequently work much better. It might then be worth re-adjusting using a screwdriver through the hole on the drum, to maximise the benefit.
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2003 HDi Handbrake - Dynamic Dave
Vauxhall recommended almost the same thing (except to not put the H/B full on, but only until resistance was felt) for the Mk3 Cavalier and Vectra-B, which also has drum brakes for the handbrake if rear discs are fitted.