Usually a nice lump hammer can be helpfull but if no joy halfords sell on their laser line range of tools a hub puller, this tool has a big bolt in the middle and is a like a bridge with 2 holes either side which locate into the wheel bolt holes on the drum, then you secure them with the wheels bolts and tighten up that big bolt in the middle which then in turn pulls the drum of the brakes shoes or the back plate.
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