A major re-adjustment of the primary Canadian Gravity Standardization Network (CGSN) is carried out based on relative gravity measurements spanning over 40 years. Anticipating that secular variations of gravity, dominated primarily by the continuing process of glacial isostatic adjustment could be detected, we parameterize the time rate of change of gravity ($dot{g}$) into the observation equation and estimate it directly from a generalised least squares solution. Time rates of change of gravity ($dot{g}$) from repeated absolute gravity observations, as well as absolute gravity values at selected sites are used as weighted constraints. The results are presented in form of a $dot{g}$ map and demonstrate that the primary CGSN, with its over 40-year-long history, can provide us with useful constraints for the evaluation/verification and refinement of post-glacial rebound models.