The problem with old pads with new rotors is that the old pads have formed to the irregularities of the old rotor surface. Now the irregularities of the old pads don't line up with the relatively flat surface of the new rotors. So now you only have a portion of the pad material contacting the rotors. You described this as having half the braking power.

Depending on how irregular the old pad surfaces are you may be able to remedy this by going through the proper pad bedding procedure several times. Hopefully the pads will reform to the new surface enough to get proper braking.

It's a good idea to go through the bedding procedure with new rotors, even if you didn't notice any problems. The bedding procedure transfers pad material to the rotors for proper braking. So when you change out the rotors you should do it again.

G/L

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