Actuation pump of a braking system for vehicles, comprising - a pump body (13, 14) which identifies a direct actuation chamber and an indirect actuation chamber filled with brake fluid and in series with each other, - wherein the direct actuation chamber houses a direct piston connected to a manual actuation member, and is fluidically connected to a first delivery line connectable to at least a first braking device, - wherein the direct actuation chamber houses an indirect piston, mechanically separate from the direct piston, and having a first end facing the direct piston and a second end sealingly defining the indirect actuation chamber, - wherein the indirect actuation chamber is fluidically connected in output to a second delivery line connectable to at least a second braking device, - and characterized in that the indirect piston comprises a first and a second head (52,56) sealingly sliding along a lateral guide wall of the indirect actuation chamber, the first head having a greater diameter than the second head and being arranged on the side of the second end, the second head facing the second delivery line, on the side opposite the second end, - wherein the indirect piston divides the indirect actuation chamber into an intermediate volume, between the first and second head (52,56), and into a delivery volume, comprised between the second head and the second delivery line, - wherein the indirect piston is provided with a control valve calibrated so that, below a threshold pressure value in the indirect actuation chamber, it fluidically connects the intermediate volume and the delivery volume to each other, and upon reaching said pressure threshold value, fluidically disconnects said intermediate volume and delivery volume and prevents such threshold value being exceeded in the intermediate volume. - a pump body (13, 14) which identifies a direct actuation chamber and an indirect actuation chamber filled with brake fluid and in series with each other, - wherein the direct actuation chamber houses a direct piston connected to a manual actuation member, and is fluidically connected to a first delivery line connectable to at least a first braking device, - wherein the direct actuation chamber houses an indirect piston, mechanically separate from the direct piston, and having a first end facing the direct piston and a second end sealingly defining the indirect actuation chamber, - wherein the indirect actuation chamber is fluidically connected in output to a second delivery line connectable to at least a second braking device, - and characterized in that the indirect piston comprises a first and a second head (52,56) sealingly sliding along a lateral guide wall of the indirect actuation chamber, the first head having a greater diameter than the second head and being arranged on the side of the second end, the second head facing the second delivery line, on the side opposite the second end, - wherein the indirect piston divides the indirect actuation chamber into an intermediate volume, between the first and second head (52,56), and into a delivery volume, comprised between the second head and the second delivery line, - wherein the indirect piston is provided with a control valve calibrated so that, below a threshold pressure value in the indirect actuation chamber, it fluidically connects the intermediate volume and the delivery volume to each other, and upon reaching said pressure threshold value, fluidically disconnects said intermediate volume and delivery volume and prevents such threshold value being exceeded in the intermediate volume.