Two-part plastic piston with excess resin receiving groove

A plastic cylinder of a master cylinder, comprising:

• a first member made of plastic and composed of a large diameter portion slidable inside a cylinder bore, and a small diameter portion extending axially from the large diameter portion for forming a stepped portion on the large diameter portion;
• a second member made of plastic and having therein a large bore portion having an inner end;
• said small diameter portion of said first member being inserted into and engaged with said large bore portion of said second member and forming a seal ring mounting groove on an outer peripheral surface of said small diameter portion between said stepped portion and the end of said large bore portion of said second member, the length of said small diameter portion in said large bore portion being less than the length of said large bore portion for defining a gap between the end of said small diameter portion and the inner end of said large bore portion, and said small diameter portion having a weld material receiving groove therearound spaced from said gap;
• and
• a welding portion between the outer peripheral surface of said small diameter portion and extending along the length thereof between said weld material receiving groove and said gap;
• whereby when said first and second members are welded to each other at said weld portion, excess welding resin will flow only into said gap and said weld material receiving groove and will be prevented from escaping to the exterior surface of said large bore portion or to said seal ring mounting groove.