H104X piston regulable pressure reducing or pressure sustaining valve is a special valve installed in pipelines of drainage system of high buildings which can reduce the inlet pressure to a certain kind of outlet pressure required. The valve relies on its own energy to make the outlet pressure maintain a set value, which means that the outlet pressure doesn't change due to the changes of inlet pressure and rate of flow. What's more, at the inlet of valve control system is a self-cleaning filter which makes suspended particles with larger proportion and diameter not enter the control system by using characteristic of flow to ensure smooth system circulation so that the valve can operate safely and reliably. It has stable system operation, high strength, long service life. The piston type is applicable to valves of more than DN450mm.