Have a look around the slave cylinder and the Flexi line to it.
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You don't need to leak much to cause this problem.
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If there are no leaks try a 2 man bleed.
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With the back end up on axle stands you can get to the bleed nipple from underneath.
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After bleeding a couple of squirts try pushing the rod into the slave whilst opening the nipple to back bleed.
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Then carry on the the 2 man bleed as normal.
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If doing that doesn't sort it and no leaks are present then it is likely the master cylinder seals are knackered, letting fluid past the plunger rather than pushing it.
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