ok to start make sure you have bled the brakes properly
(I have made a video in the tutorial section about it)
once you have done that what you want to check is what is happening at the master cyclinder, is the pedal / lever going hard? if you hold pressure on it for a little while does it slowly go down? if its going hard and not slowly going down under pressure then its probably ok.
Next go to your brake caliper, check that there is nothing obstructing the pads from moving back and foward on the slides (normally remove then and check, probably give the pads a quick ruff up on the concrete while they are out) once you know the pads can move freely reassemble everything and have the back wheel off the ground and spin it around and see what happens when you push the brakes. Are both the pads pushing on the brake disc? is the disc bending? if the problem seems to be in one of these then check that the caliper itself can slide on its mounting slide. If they seem to work fine but still arn't giving you all the power you want try adjusting the brake pedal to the correct height.
if you try all this and it doesn't solve the issue let us know what you find and I will see if I can help you further.
