question about ReadFile on pipes

i noticed recently that the helper program i.exe chews up tons of CPU time,
which seemed very strange.

the problem is this:

  while (ReadFile (pi->source, buffer, sizeof (buffer), &really_read, NULL) &&
  WriteFile (pi->drain, buffer, really_read, &unused, NULL))
    ;

the docs for ReadFile say:

The ReadFile function returns when one of the following is true: a write
operation completes on the write end of the pipe, the number of bytes requested
has been read, or an error occurs.

but this is obviously not true, at least on my win2000 system.  putting in some
printf's shows that ReadFile *never* blocks, and instead just returns 0 bytes.
adding Sleep(100) fixes things but this is a kludge.

what's going on here?