\endlinechar=-1
\show\
\bye

* 6 January 2014
C947. Space after \csname\endcsname to match other spaces. (Oleg Bulatov) @262

\documentclass[a4paper]{article}

\usepackage[T1]{fontenc}

\begin{document}

{
  \fontfamily{phv}\fontseries{mc}\selectfont
  THIS IS SAMPLE TEXT \textbf{THIS IS SAMPLE TEXT}%
}

\DeclareFontSeriesChangeRule {mc}{b}{bc}{}
\DeclareFontSeriesChangeRule {m} {c}{c}{mc}
\DeclareFontSeriesChangeRule {m}{mc}{c}{mc}

{\fontfamily{phv}\fontseries{c}\selectfont %grr
  THIS IS SAMPLE TEXT \textbf{THIS IS SAMPLE TEXT}%
}

{\fontfamily{phv}\fontseriesforce{mc}% this works
  \selectfont
  THIS IS SAMPLE TEXT \textbf{THIS IS SAMPLE TEXT}%