  Wilson's exact renormalization group equations are derived and integrated for
the relevant part of the pure Yang-Mills action. We discuss in detail how
modified Slavnov-Taylor identities controle the breaking of BRST invariance in
the presence of a finite infrared cutoff $k$ through relations among different
parameters in the effective action. In particular they imply a nonvanishing
gluon mass term for nonvanishing $k$. The requirement of consistency between
the renormalization group flow and the modified Slavnov- Taylor identities
allows to control the self-consistency of truncations of the effective action.
