A new pad on a damaged or contaminated rotor may retain poor feel, so pad history, rotor surface and evidence for paired replacement belong in the same discussion.
I don't treat “new pads” as a verdict on the whole brake setup. The decision is whether the available evidence points to dealing with the rotor too, rather than making the pad swap the end of the conversation. I prefer comparing the pad's history and the rotor's surface before settling on a paired service, without assuming that both parts automatically need replacing. When a new pad still feels poor, what evidence do you compare before deciding whether the rotor should be replaced at the same time?
I’d check rotor thickness against its stamped minimum first, then look for deep scoring, obvious distortion, or contamination that remains after cleaning. Light surface marks alone wouldn’t make me replace it. If it’s within spec and clean, bed the new pads properly before calling the setup bad; if it’s below minimum or badly damaged, replace it rather than hoping fresh pads will fix it.
A rotor can look clean and still carry contamination from the old pads. If those pads were contaminated, clean the rotor with isopropyl alcohol and a fresh lint-free cloth, then bed in uncontaminated pads; if braking is still poor after that, replacing the rotor is reasonable. I wouldn’t bin one for faint marks alone.
Poor braking after cleaning and bedding still doesn’t isolate the rotor. Check that the caliper is centered and the pads are contacting evenly before buying one.
Yes, I wouldn’t buy a rotor on that symptom alone. Check that the caliper is square and both pads are being pushed evenly; a lazy piston can leave an uneven contact patch even when the caliper looks centered. If contact is full and even but the brake still has weak bite after bedding, alignment hasn’t explained it, and then the rotor’s condition and contamination history matter.
One caveat: both pads moving under piston pressure only applies to a caliper with pistons on both sides. On a single-piston sliding caliper, the caliper itself has to move to bring the other pad in, so check that it slides freely rather than looking for a second lazy piston.
Fair correction. My wording about both pads moving under piston pressure was too loose: on a single-piston sliding caliper, the piston moves one pad and the caliper has to slide to bring the other in. Check that slide movement before calling it a lazy piston.
