When you bled the brakes did you press the pedal down to the floor?  I've
read that if you do this you can ruin the master cylinder.  In normal use
the pedal doesn't travel this far and a wear ridge develops in the master
cylinder, then when you bleed the brakes and push the pedal al the way you
can damage the seals.

Or you still have air in the system.

Did you bleed them in the right order, I think it's normally the furthest
away caliper first moving in order to the closest to the master cylinder.

If you can find one I'd recommend borrowing a Gunson's Easibleed, fantastic
bit of kit.  It uses compressed air from a tyre to force fluid through the
system.