This paper presents a new method, the HHL 91 algorithm, for calculating the network system reliability by sum of disjoint products (sdp). The main feature of the HHL 91 is its ability to properly arrange the order of minimal paths as well as to apply inversion to products of several variables. While the Abraham algorithm and its successors yield relatively short sdp forms of the structure functions of coherent network systems, this new method generates shorter disjoint products than any other known sdp method. Because the system reliability formula is considerably reduced in size, there will be a sharp decrease both in computation time and in rounding errors.