be more accurately controlled into the tubing 14 since the pressure in the injection line 16 will change when the valves 30 and or 26 are opened a pump controller and a flowmeter could be included fluidly coupled to the line 16 for example at the surface of the borehole for enabling more precise control of fluid rate regardless of changing pump resistance or fluid pressures under some conditions a pressure differential device may not be required for example a pressure differential device will not be necessary if the pressure in the production tubing 14 is high enough that the pressure required to inject fluid into the production tubing 14 is greater than the pressure required to open the mechanism 30 the system 10 is shown in fig4 and 5 incorporating an assembly 12 a and an assembly 12 b respectively in lieu of the assembly 12 the assembly 12 a incorporates a valve mechanism 30 a the valve mechanism 30 a includes a piston 48 actuatable by pressure supplied to a chamber 50 via the control leg 20 the piston 48 includes a passage 52 therethrough for selectively connecting the portions of the injection leg 18 on opposite sides of the valve mechanism 30 a when properly positioned a spring 54 is included to bias the piston 48 into a closed position wherein the passage 52 is misaligned with the injection leg 18 so that fluid can not pass to the injection port 24 the mechanism 30 a includes a plurality of seal elements 56 to seal the piston chamber 50 the passage 52 the injection leg 18 etc from each other thus it is to be appreciated that even though some degree of fluid migration may occur up the portion of the injection leg 18 downstream of the.