When two machines M₁ and M₂ are in series the output of M₁ which is the semi finished product is the input for M₂. If M₁ goes to the down state the supply or input to M₂ is stopped and M₂ goes to the down state. The idle time cost of M₂ is high and prohibitive. Hence a reserve of semi finished product in between M₁ and M₂ is suggested. In this paper the optimal reserve size is determined under the assumption the interarrival times between breakdowns of M₁ is a random variable which undergoes a parametric change after a truncation point which is also a random variable. Numerical illustration is also provided.