Abstract: An improved method for detecting small leaks in underground pressurized piping systems is disclosed. In particular, the method comprises the steps of taking pressure measurements over identical time intervals from a product line at a pressure above the pump discharge pressure while the product pump is running. Thereafter, the absolute value of the difference between successive pressure decays is calculated and compared to an acceptable value. Next, the value of the latest pressure decay value is compared to acceptable and unacceptable values reflecting acceptable and unacceptable tightness of the product line.