Steel Frame Creaks while Braking
You need to isolate this noise to identify the cause.

Its great that you can duplicate the noise without having to ride.  Try these suggestions to help narrow down the cause.


squeeze the rear of the front wheel and the downtube together with your hand.  This will replicate some of the stopping forces on your headset.


We know the brakes show the problem without being the cause.  


take the front wheel off  and push/pull the fork tines/legs back towards the downtube.  You might prefer to use some light rope in the dropouts and pull the ends back towards the bottom bracket. 


This removes the wheel as a possible source.  Also eliminates the brakes and brake mount.


hold the top tube between your thighs, the down tube between your calf muscles.  Hold the tops of your bars and push away from you.  This duplicates the previous test but from the other side of the headset.


You should not hear any clicks or clunks, and you should not feel any movement.


finally wrap an ungloved hand around the top of the headset where the steerer goes into the frame.  You should not feel any motion between the steerer and the frame, other than rotation of the steering.


The fix will likely involve removing your stem and handlebars, and cleaning/regreasing the ball bearings in your headset.  Then adjusting them to be Just Right.  At the moment I think they're too loose.  

Hopefully you've caught this problem before the bearing races have been damaged.