Preferably there is provided a bleed path for high pressure fluid to bleed from the high pressure chamber upon movement of the piston means by a predetermined maximum extent, the opening of the bleed path as a result of said predetermined maximum movement occurring relieving fluid pressure in the high pressure chamber to an extent sufficient to stop injection of fluid through the injection valve.

The present invention also provides an injecting system comprising an injecting apparatus according to the invention, a fluid pressure relief path through which fluid pressure in the low pressure chamber can be controllably relieved to permit and control movement of the piston means, and an associated fluid pressure governor means, the governor means being selectively controllable to control the fluid pressure in the low pressure chamber by selectively preventing or progressively limiting relief of pressure from the low pressure chamber through the fluid pressure relief path in response to movement of the piston means. In this injecting system, the governor means may include a flow restriction means in the fluid pressure relief path to selectively control the cross sectional area of the fluid pressure relief path, the flow restriction means having an associated drive means so as to drive the flow restriction means to vary the cross sectional area of the relief path, the governor means further including a back pressure valve located in the fluid pressure relief path, downstream of the flow restriction means, the back pressure valve being operative to maintain a predetermined minimum back pressure in the fluid pressure relief path by only opening when the predetermined minimum back pressure is exceeded.