A slitted base cavity of constant depth with a varying filling ratio 0≤R_{f}≤100% is experimentally investigated to reduce the form drag of a three-dimensional blunt body (the so-called squareback Ahmed body) at a Reynolds number Re=2.89×10^{5}. The drag reduction is achieved by a decrease of base suction (or, equivalently, the increase of pressure at the base). The plain cavity (R_{f}=100%) reduces the base suction by 22% compared to the case with no cavity (R_{f}=0). All intermediate filling ratio are obtained by the enlargement of the slits, initially having a zero width for the plain cavity case. It is shown that the gradual base suction change can be related to the level of the entrainment flux of the free shear layers developing from the rear separation and to the suppression of the transverse steady asymmetric instability of the wake. The model of the vortex formation region length of Gerrard [J. Fluid Mech. 25, 401 (1966)0022-112010.1017/S0022112066001721] is shown to provide an insightful interpretation of the drag reduction mechanism using ventilated base cavities.