backing

while riding yesterday, at one point I applied the rear brake only...and there was no tension on the foot lever, as if all the hydraulic fluid had drained out. That's probably not good, so I pulled over, tried the rear brake again, which then had normal tension and worked fine...except for a...

What would cause it to back out? Also can I just take pressure off the swing arm and tighten it back up? Sent from my SAMSUNG-SGH-I337 using Tapatalk