An implantable body fluid drainage system comprising: a metering shunt including (a) a housing with an internal chamber, (b) a movable barrier (501; 701; 901) dividing the chamber into a first section and a second section, wherein the barrier (501; 701; 901) is displaced by a differential pressure, (c) a first powered inlet valve providing a fill path to the first section of the chamber, and (d) a first powered drain valve providing a drain path from the first section of the chamber; a CSF inlet conduit configured to connect a CSF space to the first powered inlet valve; a CSF outlet conduit configured to connect the first powered outlet valve to a discharge location; and a controller configured to (a) open the first powered inlet valve and close the first powered drain valve to fill the first section to a volume defined by the barrier and chamber geometry and (b) close the first powered inlet valve and open the first powered drain valve to discharge the filled volume from the first section through the outlet conduit, wherein the first powered inlet valve is a three-way valve configured to selectively connect either the first section or the second section of the chamber to CSF inlet conduit and wherein the first powered outlet valve is a three-way valve configured to selectively connect either the first section or the second section of the chamber to the CSF outlet conduit, wherein the controller is further configured to (a) position the first powered three-way inlet valve to deliver CSF from the CSF inlet conduit to the first section while blocking the second section from the CSF inlet conduit and (b) position the first powered drain valve to discharge the filled volume from the first section through the outlet conduit while blocking the second section from the discharge conduit. a metering shunt including (a) a housing with an internal chamber, (b) a movable barrier (501; 701; 901) dividing the chamber into a first section and a second section, wherein the barrier (501; 701; 901) is displaced by a differential pressure, (c) a first powered inlet valve providing a fill path to the first section of the chamber, and (d) a first powered drain valve providing a drain path from the first section of the chamber; a CSF inlet conduit configured to connect a CSF space to the first powered inlet valve; a CSF outlet conduit configured to connect the first powered outlet valve to a discharge location; and a controller configured to (a) open the first powered inlet valve and close the first powered drain valve to fill the first section to a volume defined by the barrier and chamber geometry and (b) close the first powered inlet valve and open the first powered drain valve to discharge the filled volume from the first section through the outlet conduit, wherein the first powered inlet valve is a three-way valve configured to selectively connect either the first section or the second section of the chamber to CSF inlet conduit and wherein the first powered outlet valve is a three-way valve configured to selectively connect either the first section or the second section of the chamber to the CSF outlet conduit, wherein the controller is further configured to (a) position the first powered three-way inlet valve to deliver CSF from the CSF inlet conduit to the first section while blocking the second section from the CSF inlet conduit and (b) position the first powered drain valve to discharge the filled volume from the first section through the outlet conduit while blocking the second section from the discharge conduit.