Loosen bleeder to bleed off fluid as piston goes back in.
On my ED it is necessary to rotate them (anticlockwise???) as well as push them back down. I know there is a special tool for this but you should be able to improvise something. I think I got by with a LARGE screw driver and spanner but don't rely only on the thread alone, you also have to apply downwards force.
I had to take the caliper off the car on one side to get enough leverage. I think the piston actually comes free from the thread when the pads are excessively worn so it may take some turning before it re-engages.
Hope that helps.
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