2003 Ford F-150 Clutch not fully disengaging at times

My 2003 F-150 with 106,000 on it has occasionally been hard to shift into 1st after backing out of the driveway. Today I noticed that if you start backing up and push the clutch back down without braking, the truck will continue to want to reverse for a couple of seconds then you can feel the clutch fully disengage and it will easily go into first. Usually this does not happen and the clutch fully disengages immediately. What does this sound like to you guys? Most of the time it acts like it should, but sometimes the stick seems to want to "stick" in gear (usually reverse) and when it does that, it will be difficult to get back into gear for a few moments. Sometimes revving the engine a bit seems to help. I did check the clutch fluid reservoir and it needed a small amount of fluid to fill to the step, I checked again after a couple of days and the level hadn't dropped.