Thanks David and Alistair for your fast responses.  The magnet ring triggering the reed switches makes sense.  I'll check out Mallahat Auto wrecking or Hogg's VW repair, and if there is no spare one, I'll order that reed. In the meantime I'll have another look to make sure I order the right part.
Thanks
  ----- Original Message ----- 
  From: David Beierl 
  To: vanagon@GERRY.VANAGON.COM 
  Sent: Friday, August 06, 2010 8:07 PM
  Subject: Re: Water level sensor in camper tank

  At 10:14 PM 8/6/2010 Friday, Ed Wiebe wrote:
  >http://picasaweb.google.ca/ewieber/WestyDomesticWaterTankLevelSensor#
  >you can see the broken part.  What is it and is it easy to find a
  >working replacement.  It shouldn't be hard to solder a new part in.

  It's a reed switch.  *If* it has only two leads, any one you can find
  should work.  I have a nasty feeling, though, that it's the kind with
  three, which is harder to find.  If so, you need to be looking for an
  SPDT reed switch.  I can't quite tell from the pictures and it's been
  too many years since I've been inside one of these things.   If it's
  a three-lead one and only two leads are used, you need the pair that
  are normally closed. While you're installing it, resolder the other
  one so there's some slack in the leads -- that's why it broke in the
  first place, the twinkies didn't give it any room to stretch if the
  rod gets bent a little.  When you put it back together, make sure
  it's absolutely watertight, because a couple drops of water inside
  will make it misbehave.  The weakest point for sealing is where the
  wire comes out the top.

  >By the way, can someone explain how the unit senses the water level?

  Yes.  How much detail do you want?  Basically, the float has a magnet
  in it, and when the magnet gets near the little reed switches they
  switch.  That throws the resistors in and out of the circuit,
  changing the voltage.

  Yours,
  David