How to assemble '70s-era center-pull brake I've (supposedly) got a 1976 Motobecane Le Champion with what appears to be the original center-pull rim brakes, which unforunately are not in one piece and need to be re-assembled:


As you can see in the picture, there is a spring on each caliper arm (in the blue square) which needs to be "set" between a hook on the center piece and a hook on the arm itself. When doing this for only one side, you can simply manipulate the arm into place by squeezing it in your hand and then letting the spring settle in. However, when trying to assemble the other arm, I run into huge problems:


The spring for the inner (here unattached) arm needs to be put on before the arm itself
The two arms need to "already" be aligned together, as there is a "rivulet" on the inner plate into which a rubberized "nub" on the back of the outer plate fits (outlined in green).
However, the aforementioned spring needs to be set properly before aligning the two plates, because there is no way to move the spring into place once aligned in such a way.
Trying to "squeeze" the second arm into place doesn't work as with the first one because it's already "influenced" by the first one, since they interlock (as described above) and, in order to "squeeze" the second arm, the first arm also has to be squeezed at the same time so that the "pulls" on the end of the arm (where the cables are supposed to go) otherwise block the movement of the opposing arm.


Unfortunately, I have tiny little baby hands and so I simply can't squeeze the two arms together in a way which is non-clumsy enough to allow me to then use a ridiculous amount of strength with the other hand to contract the spring manually in order to set the opposing arm.

However, after literally hours of trying to take it apart and put it back together, I started to think (hope?) that there must be a more efficient way of doing this which I have somehow missed: After all, these brakes were designed to be serviced by humanoids with two hands. Unfortunately, the internet has not been very useful in this matter; Does no one have an idiot-proof set of step-by-step instructions for assembling this type of brake?
I finally got it to work by employing a third "hand" as such:


Disassemble both arms.
Start with the innermost arm, squeezing the arm into place so that, when it is in its "resting" position, the spring lies inside the two respective hooks on the inner plate and the arm itself.
Screw the retaining bolt for the inner arm all the way, so that the arm remains on the bike without falling off.
Grab the second (outermost) arm.
Put the retaining bolt into the arm and then push it just barely through so that you have enough space to put the washer on the other side, and do so.
Adhere the bolt to the arm temporarily by covering the head of the washer in adhesive tape and then wrapping it around the arm so that it holds the bolt in place by itself. Cut a ring in the tape inside the washer so that the bolt can be screwed in easily.
Squeeze the second arm into place in a similar fashion to that used for the first arm and hold it in that position with your hand.
In order to keep the spring from "jumping" out of place, push the retaining bolt through the washer and into the retaining plate. Hold the entire assembly together by pushing on the head of the bolt with your thumb.
With your thumb on the bolt, it is now possible to release your hand grip on the arm itself and then to use the hand to properly position the bolt into the plate and then screw the bolt securely into the plate.