The fluid pressure relief path also includes a downstream low pressure chamber 130. The fluid pressure in chamber 130, together with the force of spring 131 opposes movement of the shuttle valve 125 under the influence of fluid pressure from the chamber 120 passed to the intermediate chamber 121. However if the pressure differential between intermediate chamber 121 and low pressure chamber 130 rises sufficiently, the shuttle valve 125 will move and the head 126 will restrict the pressure relieving flow through the slot 123 thereby enabling the pressure in the intermediate chamber 121 to reduce by means of flow to low pressure chamber 130.

Interposed in the fluid pressure relief path between the intermediate chamber 121 and the low pressure chamber 130 is a selectively controllable flow restriction means 135 which comprises a needle valve 136 having a tapered nose portion 137 located in the passage 138 extending between intermediate chamber 121 and low pressure chamber 130. The needle valve 136 is selectively movable by means of electrical or mechanical control means 139 so as to enable selective control of the rate of pressure relief through the passage 138. This, in turn, enables control of the injection rate.