The specification discloses an improved shut off valve for a canister and lid assembly used as part of a medical suction apparatus. The valve includes a cylindrical plunger made of a soft rubber-like material and having an upwardly facing concave disc integrally formed on the top of the plunger. When the fluid within the canister rises to a predetermined level, the plunger floats upwardly forcing the disc into a sealing relationship covering and blocking a suction port. Alternately, the soft rubber plunger may be dimensioned for use as a plunger cap for the upper end of a hard plastic plunger.
The soft rubber plunger and integral concave disc are formed in a composite die, including a disc die and a plunger die. Rubber is injected into the composite die through a bore to form the plunger and integral concave disc. To remove the rubber concave disc from the disc die, air is injected into a bore while the disc die is simultaneously moved away from the plunger die.