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

Dumb question, are you 100% sure it's not the headset? If you temporarily overtighten it a bit, and go test the brakes, do they still jump? Obviously you can reset the headset to a proper adjustment afterwards. If it were me, I'd just like to know beyond any doubt that it wasn't an adjustment issue I was making.

Is the brake caliper attached tightly to the frame?

Are the brake arms on the caliper tightly secured? Sometimes the arms can be loose on the center bolt and cause the symptoms you're experiencing. There are different brake designs, but there should be a little locknut thing on the front, no?

No. Tektro dual pivots.

The definitive test for a loose headset is to duplicate what's happening to you.

Stand next to the bike, apply your front brake and rock the bike forward and back while watching and feeling for movement of the fork. I prefer to put my fingers on the lower headset cup and crown or crown race if it's slightly exposed and feel there.

BTW- if the headset checks out, you might not be feeling the fork move. Instead you might be feeling the brake arms move forward with the motion of the wheel and taking up the slop in the brake caliper. Some slop is normal since the arms have to be free to pivot. Better brakes are built tighter, and many are adjustable for arm play.

IMO- other than checking/adjusting your headset I'd ignore it unless it gets worse.