Doubt very much that it's a warped disc, that seems to be peoples immediate response but it's unlikely.
Definitely look towards the calipers being sticky, well worth a bit of TLC if they haven't been cleaned recently.
I had this issue for ages and for me it turned out to be a faulty braided brake line.
But if you haven't got braided lines then there's even more of a possibility it could be standard lines expanding on braking.
I wouldn't be too concerned unless it's a constant thing though!
Definitely look towards the calipers being sticky, well worth a bit of TLC if they haven't been cleaned recently.
I had this issue for ages and for me it turned out to be a faulty braided brake line.
But if you haven't got braided lines then there's even more of a possibility it could be standard lines expanding on braking.
I wouldn't be too concerned unless it's a constant thing though!
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