standard one piece

I'm working on a Standard one-piece. I'd never seen one before, but it is what appears to be a very old American Standard fixture. The water level in the tank is dropping fairly quickly. I removed the flush valve, which is just a flapper sitting on a plastic ring attached to the underside of the tank with 3 screws and flat brass brackets (just bar stock with a threaded hole). The flapper seemed to be in good condition, only slightly worn and still quite flexible. The seat also seemed to be smooth, with no wear or pitting. The rubber washer between the plastic frame and the bottom of the tank compartment seemed to be in good shape with no cracking although it is a little firm, but it still resilient. I coated it with plumbers grease thinking I might get a little better seal. I also cleaned the threads on the hold down screws and apllied teflon paste to lubricate the threads. I have tightened it as tight as I dare, yet the water level drops by enough in a couple of minutes to cause the stool to flush. I didn't closely examine the plastic ring to check for cracks, although I should have, but it appeared solid.

Did notice something in the bottom of the tank that I didn't understand. Around the perimeter of the flush valve are three "grommets" that appear to go through the bottom of the tank compartment. I don't know the purpose of these since this is a one piece unit, but could these grommets be causing the leak? What is the purpose of the grommets?

Just an FYI, but it's got a fairly new Fluidmaster fill valve. Don't know how that could possibly impact the tank compartment water loss, though.

I saw an earlier post mentioning that the flexible fill tube from the fluidmaster could be inserted too far into the ceramic well. In that case was it siphoning water out of the tank?

Any help with this "relic" would be appreciated.

Thanks,
Sherman

The Fluidmaster 400A will definitey set up a siphon it the end of the small refill tube is below the water level in the tank. Pull this up and see if it helps./

It's got a Fluidmaster 400A! I'll check to make sure the tube is above the water line. I would never have guessed that it would siphon.
Thanks,
Sherman

AS one piece

The 3 grommets are there because the toilet originally had a flush valve with three bolts that went through the ceramic. The new one has the flat "toggle" bars so the grommets were inserted to seal the holes. The replacement for that unit usually has a brass ring, but the plastic one may be satisfactory. But, I have never seen a 400A used as a replacement and still give a satisfactory flush. In fact I have only seen a couple of 400A's used in that situation, and I replaced both of them with the correct fill valve.