In this paper, an approximation method is given for the analysis of flow lines with a continuous product flow. The algorithm is an improvement of an earlier developed method. It is applied here to lines in which the machines are unreliable with exponentially distributed life and repair times. All intermediate buffers have finite capacity. The algorithm consists of repeated decomposition and aggregation steps, in which two-stage lines are approximated by a single machine. The method appears to be fast and performs well for lines which are not too long.