How does having one new rotor and one old rotor create uneven braking? Assuming that the old rotor is still within spec and in good shape.

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1987 300SDL 167k
1992 Volvo 740 140k
1990 Volvo 740 250k
1989 Volvo 240 269k

Anyone want to trade an old Volvo for an '87 300sdl?