Repairing center-pulled brakes
Brake cable should go through gap in a screw, that is locked with a nut to the holder, connecting cable pulled by the lever with, as you called, helper. Sometimes a gap for the screw in holder is not circular. If you mis-oriented screw against the hole in holder, it rests outside the hole and does not tighten the cable.

Please, add also the photo of the screw that holds brake cable on the brake arm.
It looks like you have to unscrew that screw, pull more cable and then tight it. Also put a bit of oil on the cable where it goes through circular element.

I strongly advice reading article on adjusting cantilever brake. In short:


unscrew adjusting screw on the brake lever housing by 3 turns, secure it with nut.
unscrew brake pads holders, push the pads towards wheel as far as possible (this will increase braking force), tighten it a bit
unscrew brake cable holder on the brake arm, pull it as far as possible - pads should touch the rim. Tighten the screw.
Adjust pads that they are adjacent to the rim, tighten them
screw adjusting screw mentioned in pt 1 back.


adjust brake arms position witk screws on the bottom of them.