The tub and drain in my home are from the 1930's (I think). The drain is controlled by a knob on the wall. The drain no longer stays open unless the knob is held. When I let go of the knob it clunks back to the closed position. The drain itself is a plunger style. The knob on the wall controls the plunger. It looks like gravity is pulling on the plunger which pulls on the knob. Does anyone know what keeps the drain open?
Note: This is not a common lever style drain.