In practical production lines, machines are typically unreliable and experience random breakdowns. To mitigate the effect of machine breakdowns, buffers are used to store semi-finished parts between machines. However, buffers are costly, and thus it is necessary to maintain proper buffer capacities. In this paper, we reveal an important relationship between machine reliability parameters and buffer capacities. The continuous flow production line model is considered. Two production lines are called similar if they have the same number of machines, the corresponding machines have the same processing rate, and the corresponding machine uptimes (MTBFs), machine downtimes (MTTRs) and buffer capacities are proportional. It is found that all similar production lines have the same throughput. Based on the similarity property, we illustrate how the throughput of a production line is affected by the relationship between machine reliability parameters and buffer capacities. The obtained results are useful for the design, control and management of unreliable production lines.