I have a toilet that uses a disk-like flap instead of a ball. It pivots on a hinge and "hammers" down the flap onto the drain. I bought a brand new flap assembly and now when the toilet is filling after flushing, the flap vibrates and continues until you press it down. No signs of tears on the flap or the drain fitting. Help! I used 46 CCF of water this quarter because it just continues to fail. Thanks.

Sounds like your flapper linkage needs adjusting.
With the flapper seated the linkage wants to have 1/4" of play. Too much play and the flapper doesn't get pulled back enough giving you a short flush. Too tight and it lets water seep past the flapper. I adjust the chain by the link and if it needs fine tuning, I bend the flush lever rod a bit untill I have the desired play in the linkage. One more thing about a flapper. you will see where the old flapper hooks onto the base of the white overflow tube. On a older type with no hooks the flapper has a ring that slips down over the overflow tube to the seat. If your tank has hooks, take a sharp knife or scissors and cut the neoprene ring off on the marks provided and hook the flapper on the hooks. Leaving the ring on will interfere with the flush. The water level in your tank should be 3/8 to 1/2" below the top of the overflow tube when the tank's filled. Good luck, Tom

Thanks. I'll take the "short flush" over the vibration. I'll see how this works--it tends to work for a few flushes and then mysteriously starts again. My flapper isn't connected to the white overflow tube so I can't take that advice. But not to say there isn't a small obstruction on the seat--that might have to be my next trick. Thanks Tom!