Do you have both brakes, and you're fitting them to a non-recessed frame by any chance?
If so, what I've found is the simplest way is to swap the brakes round and use the front brake on the rear so the bolt is long enough. Then drill the rear hole on the fork and use a longer (recessed style) nut that can reach the threaded part of the shorter rear bolt.
Ah yes. That is intelligent, Alex. I have several internal nuts of various lengths and may not need to do any surgery on the (modern) front fork. This astuce had not occurred to me — thanks for suggesting!
Do you have both brakes, and you're fitting them to a non-recessed frame by any chance?
If so, what I've found is the simplest way is to swap the brakes round and use the front brake on the rear so the bolt is long enough. Then drill the rear hole on the fork and use a longer (recessed style) nut that can reach the threaded part of the shorter rear bolt.