I have Tektro disc brakes on my bike, and I guess I pulled the lever when the back wheel was off. After the pads were not separating I tried to force them apart, but got no luck. Is it because there is too much fluid in the system, or I cannot separate them with the wheel off?
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.