Homeowner Forum / Plumbing / June 2008

Dear Sherlock,

  You have analyzed the problem well.  It is probably the ballcock.
The only way to start testing is to shut off the supply, take apart
the ballcock, put a rag over the top, and ease the water on.  This
will often solve the problem, as rust can accumulate right at the
ballcock washer.  If the ballcock is old, just change it with a
fluidmaster or similar brand.

   If it's not the ballcock itself, the next suspect is the shut-off
valve. First check the supply tube, shut off the water supply, and
then take the shut-off valve apart, looking for rust or dirt or
whatever.  If you can't see the problem, the messy solution is to have
someone turn the water on and off quickly in an attempt to flush out
the line.

  If these do not work, then you'll probably need to call in someone
with expertise.