Toilet Tank - Water Siphoning Out

My brother replaced all of the tank components with a Fluidmaster Kit 400AK ( Flush Valve with Adjust-A-Flush Flapper, 400A Fill Valve, Sure-Fit® Chrome Tank Lever, 3 Bolts and Gasket). When the tank filled up, I noted that it kept filling then stopping after a few seconds wouldn't stop. When it filled I also saw that bubbles were coming from the float cup. I propped up the float cup to stop the filling process and the water level in the tank dropped and it eventually emptied--I also noted that there was a steady stream of water flowing down the side of the toilet bowl which of course stopped after the tank was empty. I looked at the Fluidmaster web site and followed their troubleshooting instructions and the problem persists. Does anyone have any advice they can share?

DC_homeplumber said:

It sounds like the flapper is not seating properly, thus allowing the tank water to slowly drain into the bowl. This would cause the fill valve to run intermittently. Further, the rubber tube from the fill valve should be clipped to the flush valve and not simply inserted into it, as that will cause syphoning and the fill valve will periodically run.

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Nope - the Flapper is Seated Properly

That was the first thing I checked and the flapper is seated properly.

leak

If it is not the flapper, and just looking at it will not always identify a bad one, then it is either a flaw in the flapper's sealing surface or there is a crack in the overflow pipe. There are no other places that water can get from the tank to the toilet, and especially if it drains completely. If you let it drain to its minimum level, that is where the leak is occurring.

This tank also siphons out. The tube from the fill valve is too far down the overflow, causing a siphon.
New fill valves come with a clip to hold the tubing above the overflow.

Terry said:

This tank also siphons out. The tube from the fill valve is too far down the overflow, causing a siphon.
New fill valves come with a clip to hold the tubing above the overflow.

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I dig the homemade float.