A crosshead piston assembly has a piston body with an upper head portion formed with a plurality of piston rings and a lower stem portion formed with a pin bore. A return passage for the fluid from the chamber of the consumer is of such a configuration and arrangement that it can be opened and closed by deflection of the diaphragm, wherein the diaphragm is of such a configuration that, when the fluid flow in the feed passage falls below a given value, the return passage is opened. The redundant or secondary communication path may be a different line of sight path to the same or a different transmitter, or may be a lower frequency communication path in different embodiments of the present invention. With external dc voltage fields, whose strength or penetration into the interior of the rod system varies along the axis of the system, the ions can be collected or propelled along the axis of the system.