I'm riding an old steel road bike with chromoly fork, with new-ish tektro dual pivot caliper brakes.

Whenever I apply the front brake, I can feel the front fork "jump" backwards, as if the brake is pulling back on the fork. It is not a "shudder" with continuous multi-directional movement, just a momentary "jump" in one direction (backwards) when the brake is applied.

I've checked my headset, and front hub and both are adjusted properly.

I acquired this frame second-hand, and it may be possible that the headset is using mismatched cups, but there is no perceptible play between the cups that I can notice when I'm off the bike. I have an extra threaded headset I could replace it with, but I don't want to go to the trouble unless I know this is the problem.

Any one have any ideas of what this could be?

thanks