My rear brakes are tight and stop the bike if I push it in my driveway. But as soon as I take it for a ride the rear brake pedal depresses all the way down and barely stops the bike. What could be the problem? I have plenty of fluid. Why would rear brake have less pressure while driving?

This started after I dropped my bike on its left side. It rested against a wooden railroad tie (while parked) and didn't cause any damage to the bike. Also nothing came in contact with any brake parts like lines for caliper. I'm only mentioning this because as soon as I road off I my rear brakes felt like they lost pressure.