The mechanisms which prevent sealed-off operation of Cu HyBrID lasers have been investigated by measuring the transient behavior of laser output power, Cu2S 1/2 density, CuBr density and the voltage-current waveforms when buffer-gas flow is stopped during otherwise normal operation. The laser output, Cu2S 1/2 density and CuBr density decay synchronously upon ceasing the flow, and fall to negligible levels within 60 s. The observed rate of decay, and the measured change in voltage-current waveforms during this period, are consistent with the calculated loss rate of Br from the plasma to the cold end-regions due to diffusion. During normal laser operation (i.e., with gas flow), this diffusion loss of Br occurs at a rate much faster than that possible due to the entrainment action of the gas flow, and approximately equals the rate at which HBr enters the laser tube with the gas flow. It is concluded that the principal role of buffer-gas flow is to continuously source Br species into the plasma to compensate for the Br diffusion loss. The implications for the practical operation of Cu HyBrID lasers and for the development of sealed-off Cu HyBrID-type lasers are discussed.