FYI, Brake cables are thicker than the derailleur cables for obvious reasons. I would do this only as a temporary fix as these single ended straddle cables are available online (as are double ended [with non-Mafac ends]). I would assume you have Racer calipers as the Competitions have round cable ends on each end(and are about $50 apiece now!). FYI, the new ones can be modified to fit Competitions.