Flush water will not stay up in toilet

Ronald Bindhammer same answer as on a previous question about this. "This may be a difficult question to answer since we do not know the make and model of your toilet ;-) Remember that the water stays at a given level because of the u-bend, which is called a trap, within the porcelain of your toilet. The shape of the bend will determine the height of the water level. So, it is possible that that there is a crack in the trap of the toilet its self and the water is slowly running down the drain (lucky you ). The other, and actually more common issue can be a clogged or improperly installed vent. One way you can check for a plugged vent by flushing the toilet, When the toilet then fills and the tank shuts off the water to the toilet, turn on the hot and cold water on the closest fixture to the toilet, most likely your sink or the tub. If the water level moves in the toilet you have a clogged vent. The change in the level will happen within minutes. Check the vent where it sticks up through the roof and look for obstructions. You can use a snake to snake out the vent stack or flush it with a garden hose (could be messy). The other problem can be a partially clogged downstream drain line. That could siphon the water out of the toilet. give your system a good cleaning with a snake and see what happens. Hope this helps, good luck.