Water Level in Toilet Bowl

Right after flushing the toilet, the water level is much lower than normal. Gradually it does get higher, though not to a normal level. When flushed, the level rises, then the water goes down the drain. I have a Fluidmaster 400A unit. The rubber tube to fill the bowl appears to working properly and the water height in the tank is fine.
Any ideas how to fix this?
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nr Wrote:

What's "normal"? Has the toilet changed it's operating characteristic over the time you've had it? To
-- tomeshew
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Normal is what had been happening for the two months I've been in this house. No changes were made to the toilet before the low level started.
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Why is my tank filling slowly or not at all? http://www.fluidmaster.com/troubleshooting.html#q8

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Isn't the tank, it's the depth in the bowl.

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Perhaps a plugged vent stack? It is continuing to pull water down after the vacuum should have been broken by the bowl emptying. You could also have a slow drain. If the water backs up to the vent you get the same symptom. That will usually happen where the horozontal runs are long like under a slab house. Get a white bucket full of water and "chase" the flush to see if it backs upon you.
wrote:

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