Don't bleed it without the ability to replace fluid. There is very little in the system. You may just need to clear a bubble.
First, raise the reservoir up as high as possible and work the lever to encourage the bubble to migrate back to the reservoir and/or take of the slack you introduced when you reset the pistons. If it doesn't firm up and stay there, step 2 can be "Burping" the brakes by opening the reservoir lid at the highest point and slowly working the bubbles out. This may not require any additional fluid if done carefully. Google some videos of it before you try.
Option three is to get a kit and properly bleed it with additional fluid to make up for any that was lost.