accomplished by positioning a shaped control rod 64 in vent passage 65 this rod is moved by lever 41 which moves in response to water level in the tank when vent passage 65 is open it permits system pressure to enter the valve chamber 61 and the valve will be closed passage 66 being closed when passage 65 is closed and passage 66 is opened the diaphragm will move upwardly to open the valve this is a well known operation whose details are unnecessary to an understanding of this invention what is important is that when the cap is attached to the base the workings are in place and the valve is operable the objective of this invention is to keep particulate solids out of the valve beyond the valve seat for this purpose a screen 70 is provided to which flow control 71 is attached screen 70 is a body 70 a formed as a hollow tapered structure with a peripheral wall 71 having a plurality of axially extending slots 72 of a size chosen to exclude undesired solids a collar 73 on the screen fits snugly in wall 74 of the riser a socket may if desired be provided axial slits 75 are formed in the riser wall radial protuberances 77 are formed on the screen body in the illustrated embodiment these are formed as fingers which are reversely tapered so that when they have been moved past a tapered portion 78 of wall 74 of the riser the screen can be rotated and they abut it this will prevent axial removal of the screen unless and until the screen is turned again to align the fingers and.