Have you changed over your brake fluid from the china stuff ??
Get yourself some dot4 fluid and some 5mm Inside diameter clear tubing and check out this video. YouTube - Pitbike brake bleeding
Theres tones of usefull information in the Mini Tutorials & How To's - Mini Dirt Bikes & Pit Bikes Forum part of the forums ... check it out, read up. Youll prevent alot of future problems happening etc. The china's especially need work/tuning all the time.