Piston flushometers work well with a weak system gpm or in other words, a low pressure system generally within 20-30 psi range.  VMS stocks both closet and urinal pistons and parts for both manual and electronic sensor flush valves.  Whether you are looking for the piston cover, molded cup, spring, gasket, disc, the complete assembly or other piston flush valve product, look no further than Value Maintenance Supply.