The Capillary Pumped Loop (CAPL) experiment, consisting of four tubular evaporator elements configured in parallel, a dedicated starter pump, four condenser elements, and a heatpipe radiator, was tested in microgravity onboard the Space Transportation System (STS). All twenty attempts at starting the system from the flooded state using a ground-proven procedure were unsuccessful. These unexpected events are attributed to the failure of the startup procedure to clear the liquid flooding the vapor spaces of the evaporator elements before heat was applied to the evaporators. Emptying of these vapor spaces prior to the application of a heat load had been found necessary to prevent a postulated vapor blow-by mechanism that can potentially lead to arterial depriming by injection of bubbles into the liquid core of the capillary pumps. During ground testing, gravity-induced drainage was instrumental in drawing liquid out of the vapor spaces of the evaporator elements, which were located above the rest of the capillary pumped loop system. In microgravity, the capillary barrier formed at the entrance of the evaporator vapor spaces of relatively small effective diameter arrested the inflow of vapor generated by the starter pump forcing it to flow toward the condenser elements along hydrodynamically less restrictive vapor lines of larger diameter. The concept of a Capillary Vapor Flow Valve (CVFV) is advanced in this investigation for insertion into the vapor flow path immediately downstream of the evaporator vapor manifold. Its use can temporarily arrest the flow of startup vapor toward the condenser until all vapor spaces of the evaporator elements are cleared of liquid.