I'm trying to repair flush valve on an old american standard one piece low profile model. The valve has one of those balance devices made up of two plastic tubes at right angles, one sealed and full of air, and the other open on one end and drilled on the other. What happens is, the toilet flushes ok, then the valve finally closes most or all the way about 5 seconds later, then it kind of floats up off the seat and bobs along as the tank starts to fill, sometimes never closing again. If I apply light finger pressure after the flush, the valve seals and the tank fills, only to leak over several minutes and then the tank fills again. I find no nicks or dings on the valve seat, and the "flapper" is new. Can this be repaired? Should I drill out the hole so the valve closes earlier? Attach weight to the valve so it gets held down after flushing?

Irony here: The condo where this toilet is, and another just like it, was once owned by the president of American Standard. He had the toilets installed when he owned the place.