Ironically, I got oil on my rear brakes on my Kinetic too when I changed the oil in the crankcase. I sprayed some carb cleaner (it's all I had) around the rear brake area. I also rode through some puddles of water and hosed the area around the brake with a little water. Then I rode the 'ped down the road a bit to help dry it, using the rear brake (although it would not stop the ped). I did this every day for several days. Just be careful and keep your hand on the front brake until you begin to regain your rear brake power!
It took about 2 weeks before I regained most of the braking power. 3 weeks later it was back to normal (at least as far as i can remember).