In a wide class of composite models of quarks, leptons and W-bosons the existence of isoscalar weak vector bosons Y or YL is predicted. They either couple to the weak hypercharge current jμY or its left-handed part jYμL=jYL and have a mass in the few hundred GeV range. The signatures of such particles at future hadron-hadron colliders is studied by means of an effective lagrangian incorporating vector dominance. Quantities relevant for detecting and studying isoscalar weak vector bosons turn out to be sensitive to the mixing strength λY of the Y- or YL-boson with the photon. The Y or YL are expected to be produced abundantly at multi-TeV colliders. The Tevatron collider will be able to see a YL-boson if its mass is <=400 GeV.