positioned between pressure regulator 44 and cylinder 12 valve 138 is urged to a first or non operating position by a return spring 140 and the valve has a push button 142 to operate the valve valve 138 would be housed in the injector in the position such as valve 100 with actuating button 142 projecting similarly to actuating button 104 in the first or unactuated position of valve 138 both the right and left sides of piston 22 are vented to atmosphere and the supply line from pressure regulator 144 is dead ended at the valve that state of the valve is shown in fig8 when button 142 is depressed to actuate the isotope injector the valve moves to its second position where pressurized fluid is delivered to the right side of piston 22 while the left side of piston 22 is vented thus causing piston 22 to move to the left to push the plunger of the syringe for injection the operator of the injector retains control over injection during the entire injection stroke if the operator releases the actuating pressure from button 142 spring 140 will automatically return the valve 138 to its unoperated position whereby both.