The fluid pressure relief path preferably includes a pressure compensating means which includes a restriction and varying means for varying the size of the restriction in response to changes in fluid pressure downstream thereof, the varying means being operative to reduce the area of the restriction to maintain a predetermined pressure downstream of the pressure compensating means. The pressure compensating means may comprise a chamber which communicates with the low pressure chamber, the pressure compensating means further including a shuttle valve responsive to the pressure differential between the fluid pressure in that chamber and a point further downstream in the fluid pressure relief path and being operative in response to an increase in the pressure differential to reduce the area of the restriction and thereby retard pressure relief from the chamber to the point further downstream.

The injecting system may further include a controllable damper means in communication with the fluid pressure relief path, the damper means including a movable damper member responsive to a pressure increase in the fluid pressure relief path to yield so as to thereby relieve pressure in the fluid pressure relief path, the damper means further including an adjustable limiting means associated with the movable damper member to controllably limit the extent of yielding movement, the limiting means thereby effectively determining the pressure relief provided by the damper means. The movable damper member may comprise a resilient damper disc which defines one wall of a chamber which is in communication with the fluid pressure relief path, the limiting means comprising a limiting stop which is adjustable so as to be contacted by the damper disc.