I've been having a problem with the back brake, I will be riding along and go to brake and the pedal will not move and I get no braking at all than it will come out of it and work again. It feels like the piston in the caliper is stuck. Today I had to ride the last 5 miles home with it not working and just as I was within 3 blocks of the house it started working again. Any ideas what might be causing this? I am thinking I am going to replace the caliper and start from scratch.