Abstract: Pipeline plugger has a piston (22) on its push-pull rod (16) in a cylinder (23) the lower end (25, 26) of which is sealed to the swivel member (24) in which the push-pull rod is sealingly slidable, the upper end (29) of the cyliner (23) is slidably sealed on the push-pull rod, a vent hole (30) to atmosphere is provided from the cylinder (23) adjacent the swivel seal (15), and a bleed passageway (31) is provided in the push-pull rod from adjacent the plug (18) to just above the piston (22), whereby, when the damper type valve (12) is opened, gas admitted into the chamber (14) is bled through the passageway (31) to above the piston (22), thus applying a downward load on the piston annulus to counterbalance the upward gas load on the push-pull rod (16), so that the plug (18) can then be pushed down easily into the pipe (11) through the valve (12), the branch (13) and the hole (19) in the pipe.