We re-analyze published proton beam dump data taken at the U70 accelerator at IHEP Serpukhov with the ν-calorimeter I experiment in 1989 to set mass-coupling limits for dark gauge forces. The corresponding data have been used for axion and light Higgs particle searches in Refs. \cite{Blumlein:1990ay,Blumlein:1991xh} before. More recently, limits on dark gauge forces have been derived from this data set, considering a dark photon production from π0-decay \cite{Blumlein:2011mv}. Here we determine extended mass and coupling exclusion bounds for dark gauge bosons ranging to masses mγ′ of 624 MeV at admixture parameters ε≃10−6 considering high-energy Bremsstrahlung of the U-boson off the initial proton beam and different detection mechanisms.