Antique tub drain repair

I have a 1916 home which has a problem with the drain/drain lever for the tub. there is a cam type operation to open and close the drain in the tub. the handle in the tub to drain it rotates 90 deg right and left to open and close unit. Presently it only will hold the closed position, nothing appears to hold it open to drain, we have to hold it by hand until tub is completely drained. Please help. The plumbers within 50 miles of us have never seen this type of unit. Thanks, Dale Robideau, Olivia MN 320-522-1818