Disc brakes not separating with force
You normally need to take the pads out and lever the pistons. The pistons have a tight tolerance and jam easily. 

Its possible to do it with the pads in but you need a narrow enough tool to get all the way between the pads, that is strong enough to put the force need to retract the pistons. You also risk damage the pads. 

There cannot be too much fluid unless you bleed the brakes with the pistons extended. The fluid that pushed the pads out will return the reservoir, however this needs a happens quite slowly- you need to apply a gentle force to the pads for a long time rather than a brute force it. 

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