The temporary pressure drop observed during post-peak dilatant slip in axi-symmetric testing of a porous aeolian sandstone is modeled using a simple function of opening caused by sliding along the fault surface. Results suggest dilatancy of 0.02 mm accompanying frictional sliding, less than 10% of the typical grain diameter of 0.25--0.50 mm. The rapid void opening, under nominally undrained conditions at constant upstream flow rate, causes a strong 'suction pump' effect. A theoretical maximum pressure drop of 0.09 MPa or more, consistent with the experimentally measured results, can occur without a change in permeability properties.