port 9 to which is connected the hydraulic line 2 the open end of the cylinder 4 is closed off by a cylindrical cover 10 a piston 12 slides in the cylinder 4 the piston having a piston rod 13 which slides in a guide bush constituted by the cylindrical cover 10 the piston rod 13 is sealed with respect to the cover by a seal 11 the piston rod 13 has a spherical head 14 which is pivotally mounted in a joint socket not shown which is provided for example on the rod head of the guide linkage of the advance mechanism of an associated roof support unit a bore 16 is formed centrally in the top 15 of the cylinder 4 the bore 16 communicates with the port 9 and therefore with the hydraulic line 2 by way of a duct 18 which is formed in the wall of the cylinder 4 that is to say in the spherical cap 6 and the shoulder 7 a metering cylinder 17 is fixed to the bore 16 the metering cylinder being slidable in a bore 19 formed in the piston rod 13 the metering cylinder 17 is sealed with respect to the cylindrical working chamber 21 of the ram 1 by an o ring 20 a metering chamber is formed within the metering cylinder 17 by a metering piston 22 when the ram 1 is retracted pressurised hydraulic fluid.