The main seal for the SPS hole is a piston hole seal ring made of special PTFE resin combined with NBR rubber. This design minimizes the size of the mounting trench. The special PTFE resin material can avoid the crawling problem of the sealing ring used in the hole of the U-ring formation. The problem of extrusion clearance between the groove wall of piston and the inner diameter of cylinder can also be solved. Because it is a separate seal, there is no pressure storage problem between the sealing rings. The O-ring at the bottom provides the required support. The chamfer of the top resin reduces extrusion, and the slope of the sides reduces through-through leakage.