  We present new measurements of the magnitude of the main sequence turn off
and the red giant branch bump in the luminosity function of a sample of
Galactic globular clusters with updated estimates of [Fe/H] and [$\alpha$/Fe],
employing photometric data collected with the Advanced Camera for Survey on
board the HST. We compare measured and predicted values of the magnitude
difference between these two features, a rarely employed diagnostic of the
internal structure of low-mass stars at the beginning of their red giant
evolution. Our analysis discloses a clear discrepancy between theory and
observations, the theoretical red giant branch bump magnitudes being too bright
by on average $\sim0.2$~mag. This corroborates results from the more widely
studied magnitude difference between horizontal branch and red giant bump,
avoiding the well known problems associated to the determination of the
horizontal branch level from colour magnitude diagrams, and to uncertainties in
the luminosity of horizontal branch stellar models. We briefly discuss several
potential solutions of this discrepancy.
