After replacing the broken rubber washers on my toilet tank, it wouldn't fill up (water would run but it was leaking out of various spots in the old fill valve) so we replaced the fill valve with some old Fluidmaster kit (Fluidmaster 404LG). It filled up just fine without leaking. No leaks anywhere, water in toilet bowl doesn't visibly move. I flushed the toilet once and it refilled. Later went back and went to flush again but the water had drained out. I checked the flapper and it seemed to be fine. If I tap the flush lever it triggers the fill valve to start filling the tank, but then over time the water drains out somewhere and it has to be refilled every time someone wants to flush. Not sure if it is a crappy fill valve or what. The center gasket between the toilet and tank looked in rough shape but we were waiting on the replacement to arrive. It has arrived now, but we haven't installed it yet. I don't know if that would make a difference. If it was a problem with the flapper, then the fill valve should just keep refilling when it loses water, but it doesn't. Any idea what the problem is? Bad fill valve? Or maybe it needs adjustment? Someone recommended Fluidmaster Performax for the Toto Drake toilet (which is what I have) but I'd like to hear some ideas on possible fixes before replacing the valve again.