On my Shimano BR-M595 hydros, which I got in 2010 for my Kona Dew Plus, in recent months I have noticed that brake power has gotten really shoddy. One example of this, at the start of any ride, whether it is recreational or commuting to work, the brake pads don't have much grab on the rotor, and I have to press really hard on the levers to get at least moderate stopping power. It is only after making several hard stops down inclines that I begin to notice improvements, and after the ride is over for the day, for some unknown reason, the braking power goes back to what it was before. It's strange, I even tried to solve the problem with new pads and sanded and cleaned the rotors good, and I haven't noticed any leaks from the pistons. As a result, I'm really confused. What could I do in this situation to make the braking power better?

Oh, and I have bled the brakes once a year too to make sure that the lever pull was stiff, and the they still are.