There a few things you can try.
First, you may have already tried toeing in the brakes. This usually fixes the issue, but sometimes it does not.
Toeing the brakes simply means adjusting the brake pad so that the front part of the pad contacts the rim first when the brakes are applied. If you looked down on them, they would appear slightly "pigeon-toed". Here is an illustration:
Pads are often adjusted in the parallel position, but this can cause squealing. The "toe-out" position is even worse.
Last, you can try scuffing the pads on the sidewalk.
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