Fluidmaster 400 leaks, please help me to understand it

Discussion in 'Toilet Forum discussions' started by MaxBlack, Aug 24, 2011.

  1. MaxBlack

    MaxBlack Member

    Feb 11, 2007
    TX Hill Country
    Brand-new model 400 filler and it admits water at the top in a trickle. Water being the precious commodity it is, I have to stop the trickle.

    1. The black seal therein (242) appears in perfect shape, no dings or nicks

    2. Same for the white (nylon?) below, no nicks or cuts

    3. Water is rainwater, filtered and perfectly clean

    So it appears to me that the water is coming-up the center of the seal, which I assume is supposed to be sealed itself by the stainless steel wire that connects to the lever arm and float. I applied a dab of "lube-tube with silicone and PTFE" to the hole in the center of the seal that the stainless wire protrudes through, but it still leaks.

    I can take the whole shebang back to Home Depot for a new one, but guess I will buy a new seal or two first cuz that's easier.

    But it puzzles me how this stainless shaft, which of course protrudes thru the entire 242 seal, is supposed to achieve a perfect seal and stop the waterflow? Is water pressure against the underside of this seal supposed to cause it to distend in such a way as to make a watertight seal around the metal shaft?

    I'm not understanding this thing--someone help please! :eek:

    P.S. I have looked more closely at both the stainless pin and the 242 seal (with a magnifying glass) and can see that there is magic at work. The "pin" has both serrations and a "waist" to it, and the seal has extremely tiny holes on either side of the protrusion that enters the top of the valve. It probably was not wise to apply any lubricant to it, and I'll certainly clean-up the pin before putting-in a new seal. Still, I hope someone w/a better understanding of how this thing intends to work will post here. I can bearly spel "fluid dynamics"!
    Last edited: Aug 24, 2011
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    What is your water pressure? If it is too low or too high, it may not seal properly.
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  4. MaxBlack

    MaxBlack Member

    Feb 11, 2007
    TX Hill Country
    Pressure is normal i.e. 40-60psi and I do have another toilet at the same level w/the FM400 and it works just fine. Indeed our swimming pool has a filler that's a 400-style Fluidmaster and it's worked fine for 8 years.

    I guess this thing is just very sensitive to manufacturing tolerances. It appears to me that when the toilet is filling, some small amount of water leaks-past the serrated tip portion of the lever rod, such that when the lever comes up and the rod goes down there is now water above the thing equalizing the pressure above & below and sealing-off at the bottom of the 242 seal (and the rod is smooth at this position).

    I took everything apart and cleaned the lube-tube off'n the rod and the seal etc. with rubbing alcohol and put it all back together and it seems to now be fine. I'll buy a couple more seals to have on hand as it does appear these need to be "perfect" in every way to function as designed.
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