Any packing will weep or bleed a certain amount of gas. Minor amounts of weeping will occur due to gas forced into rod pores or into the lubrication film. This gas is released when the rod comes out of the high pressure portion of its stroke. In some cases, dissolved gases reduce the effectiveness of the lubricant, possibly resulting in increased friction. More commonly, leakage occurs due to incorrect fit of packing rings to rod and disturbances along the sealing surfaces.
Excessive leakage, if permitted over long periods of time, will cause deterioration to the point that it will become difficult to correct. Even new packing cannot be expected to seal adequately if the surface condition of the piston rod is poor or rod runout is excessive. For any given service, factors such as rod undersize or oversize, surface finish, taper and runout significantly influence the degree of leakage.