That's a common observation from those installed disc break first before installing rear set. One reason is you can put less pressure on the master using rear set pedal compared with stock foot rest. You need to bleed it thoroughly making sure all air bubbles are removed to make your break stronger.SIR CAN I ASK WHAT IS THE CAUSE OF MY WEAK REAR BRAKE AFTER INSTALLING REARSET AND ALREADY CONVERTED THE REAR WHEEL TO DISK BRAKE. WHAT IS THE SOLUTION??