For the last five years I have had a model pf2 in one of my toilets. It works when it wants to but won't always fill the pressure tank and the water will keep running. Make a long story short, I purchased a complete flush valve system and installed according to directions. After filling the tank to proper level I flushed but the water would not flush out of the bowl. It would run out of the tank but not enough to empty the bowl. I tried to use a plumbers helper thinking it may be air locked, but still nothing. What could be the problem?