in their entirety by reference body 14 defines main openings 42 and 46 and a hollow interior region therebetween water or other fluid thus may flow through body 14 from opening 42 to opening 46 or vice versa preferably however assembly 10 is used with a suction type apc 41 in which case main opening 42 forms an inlet to body 14 and main opening 46 constitutes an outlet also shown as included as parts of lower section 38 are at least one bypass opening 50 and one or more auxiliary openings 54 bypass opening 50 may but need not have regular shape indeed as illustrated in fig1 2 and 4 5 bypass opening 50 preferably is shaped irregularly with its nominally lower boundary 58 being longer than its nominally upper boundary 62 nominally left boundary 66 being straight and nominally right boundary 70 being curved persons skilled in the art will recognized that even if shaped irregularly bypass opening 50 may be shaped other than as depicted in fig1 2 and 4 5 beneficially though the irregular shaping of bypass opening 50 may help maintain generally constant flow through main opening 42 when the bypass opening 50 is open this is because pressure differential change is a function of the square of flow rate rather than a linear function thereof as piston 18 moves longitudinally within body 14 the effective size of bypass opening 50 see e g fig4 a c likewise changes non linearly auxiliary openings 54 allow typically higher pressure pool water to.