Quick connect tube couplings are configured for use with unstepped bores in conventional fittings, valves, manifolds and similar devices. The couplings each include a tube support which axially abuts a brass sleeve utilized to retain the coupling in the bore. The tube support further includes a stepped portion which locates a primary O-ring seal for sealing directly with a tube connected by the coupling and a secondary O-ring seal which seals with the wall of the bore. In accordance with one embodiment of the invention, the tube support is molded of plastic and, in accordance with another embodiment of the invention, the tube support is brass in order to accommodate tubes with thicker walls.