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[..]

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[hubcapsc]

If it is not too late you could use hydraulic preasure to
push the pistons  out some and clean them... then maybe
remove the reservoir cover and see if that makes it
easier to push them all in. I always watch the open reservoir
to make sure it doesn't overflow when I'm pushing the
pistons in, but I not sure that would ever really happen...

-Mike

[Firefight100]

yes you have to remove the wheel first.  A c clamp on the old pad will reseat the pistons in.