The Tl2Mn2O7 pyrochlore system, pure or doped at the Tl position (Cd, Bi, Sc) or at Mn position (Sb, Te, Cr, Pb), present interesting magnetotransport properties characterized by the Colossal Magnetoresistance (CMR) effect. Our data for Tl2Mn2O7 and Tl1.9Bi0.1Mn2O7 indicate a decrease of the ordering temperature, measured from resistivity and magnetic susceptibility up to 70-90 kbars, with a clear increase of T-C for higher pressures (up to 120 kbars). By contrast, in the case of Tl2Mn1.8Sb0.2O7, T-C increases with pressure in the explored pressure range. A recent theory from Nunez-Regueiro and Lacroix explains this dependence in terms of the existence of a second coupling mechanism between Mn ions, mediated by the conduction band (Tl (6s)). The doping at the Mn position (Sb) strongly increases the charge carrier density, so that this second coupling mediated by the conduction band, is strongly enhanced, in such a way that T-C increases with pressure.