1988 Dodge Lancer power steering

Well to start with, I replaced the rack and pinion a month or so ago, Flushed the power steering pump and lines and filled the pump with new fluid, everything has been good ever since. I should say that I over fill the reservoir a little and just left it that way.
Well over the this last weekend I left the lights on and ran down the battery. So, I pushed it out in the street and let it roll down along the curb to wait for a jump. I only turned the wheels enough to maneuver it to a parking spot. As I was waiting I noticed a leak. I found that the power steering fluid had backed up into the reservoir and was coming out from under the cap. Can you all tell what caused this to happen and what I need to do to make sure this don't happen anymore