The valve on the bottom, if you know where it is is your pump valve/
somtimes it sticks. Take it out and clean as it will have hard water sluge on it which won't let it slide. Be sure it is very clean fine steel wool is great Lube it with lithium grease. It should work perfect. Good Luck and let me know if this helps or you need help again..

Either it's a chemical injector or pressure regulator really need a pix would help