You may need to remove the pistons from the calipers, clean them
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& clean out the grooves where the seal
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& rubber boot sit in the cylinders Ian
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You'll most probably need to buy a new kit of seals
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& rubber boots to reassemble them
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I'm in the process of a similar job at the mo
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Rust forms in the top groove, behind the edge of the boot and makes the pistons stiffen up
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It can be a biggish job, perhaps you could refit your original single pot calipers while you do the ST185s
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Presumably they are twinpot 185s
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?
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I can take you step by step through how I did mine
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Peter