A research program is described which was carried out to establish the causes of interseal pressure occurring on pistons or in glands of hydraulic equipment when using pairs of seals with back-up rings. Three fractional factorial test programs are described using various configurations of seals and back-up rings, the results indicating several important geometrical factors affecting the inward pumping of the seal. The results indicated the importance of the inlet film profile of the seal. The addition of a back-up ring to the low pressure side of the seal effectively alters the inlet profile and reduces or eliminates the inward pumping of the seal.