Abstract: An apparatus for filling a liquid friction pump with a liquid comprises a metering pump and plug connector means for connection to the liquid friction coupling. During a testing of the filling ratio of the liquid friction pump it may be necessary not only to supply viscous liquid to said coupling but also to withdraw viscous liquid from such coupling. For that purpose the plug connector means are preceded by a manifold, which communicates with two nonreturn valves, which normally prevent a flow of liquid from said manifold through said valves. A discharge line from the metering pump communicates with a first of said nonreturn valves. The second nonreturn valve is incorporated in a return line leading from said manifold to a reservoir, from which the metering pump is adapted to suck liquid. Said second nonreturn valve is adapted to be opened preferably by the application of pneumatic pressure for a flow in said return line from said manifold through said second valve to said reservoir.