  We introduce a new parameter {\Delta}{\xi} - the difference in magnitude
between the red giant branch (RGB) bump and a point on the main sequence (MS)
at the same color as the bump, the "benchmark" - to estimate the helium content
in old stellar systems. Its sensitivity to helium is linear over the entire
metallicity range, it is minimally affected by age, uncertainties in the
photometric zero-point, reddening or the effects of evolution on the horizontal
branch. The two main drawbacks are the need for precise and large photometric
data sets, and a strong dependence of the {\Delta}Y/{\Delta}{\xi} slope on
metallicity. To test the {\Delta}{\xi} parameter we selected 22 Galactic
Globular Clusters (GGCs) with low foreground reddening, a broad range of iron
abundance and precise, relatively deep, and homogeneous multi-band (B,V,I)
photometry. We found that the observed {\Delta}{\xi} and those predicted from
{\alpha}-enhanced models agree quite well if we assume Y=0.20. Comparison with
canonical primordial helium content models (Y=0.245, {\Delta}Y/{\Delta}Z=1.4)
indicates that the observed {\Delta}{\xi} values are systematically smaller
than predicted. The outcome is the same if predicted parameters are based on
models that take into account also CNO enhancements and becomes even larger if
we consider He-enhanced models. These findings suggest that current stellar
evolutionary models overestimate the luminosity of the RGB bump. We also found
that including envelope overshooting can eliminate the discrepancy, as
originally suggested by Alongi et al. (1993, aaps, 97, 851). The {\Delta}{\xi}
parameter of GGCs, in spite of the possible limitations concerning the input
physics of current evolutionary models, provides an independent detection of
pre-stellar helium at least at the 5{\sigma} level.
