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New theoretical evaluations of the RGB luminosity function 'bump' and theZAHB luminosity covering the range of metallicities typical of galacticglobular cluster are presented. The variation of the theoretical RGB bump andZAHB levels due to the metallicity, original helium content, mixing lengthvalue, age, mass loss, bolometric corrections, opacities and equation of stateadopted in the evolutionary models is also discussed. These new prescriptionshave been taken into account for casting light on a longstanding astrophysicalproblem connected with the Red Giant Branch evolutionary phase, namely thediscrepancy between the observational and the theoretical luminosity of RGBbump. A sample of globular clusters with accurate evaluations of the bumpluminosity and spectroscopical metallicity determinations has been selected.The Zero Age Horizontal Branch luminosity at the RR-Lyrae instability strip hasbeen evaluated as accurately as possible, and the observational luminositydifference between the RGB bump and the ZAHB has been compared with thetheoretical values. It is shown that there is no significant disagreementbetween observations and canonical stellar models. The possible applications ofthis result are also briefly discussed.Comment: 10 pages, 5 figures, Tex file, mnrass.sty style included. To appear  in MNRA

A critical investigation on the discrepancy between the observational and the theoretical red giant luminosity function 'bump'