Rear brakes - Mondeo 2

It's the second time that the left rear brake of my Mondeo jams (sorry if my english is far for being perfect). I mean that it continuously brakes and, of course, the wheel becomes warn. When I go to the garage,
it takes less than one hour for them to repair this... Does anyone knows what do they do exactly. I think that it must be something very simple and I would like to do it myself in order to save time and money.
Thanks, JBK
PS: Mondeo 2 - 2003 - tdci - 65000km PPS: I bought it 18 months ago (26000km).

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