An infusion system, comprising: a drive assembly (5, 205) comprising an intravenous bag (30, 230); and a drive mechanism configured to apply a force to drive fluid from the drive fluid reservoir through a length of tubing (10, 210); and a hydraulic assembly (15, 215) comprising: a first fluid chamber (35, 235) having an inlet port configured to be in fluid communication with the drive fluid reservoir through the length of tubing; a second fluid chamber (45, 245) having an outlet port configured to be in fluid communication with a patient extension set (20, 220); a flexible, movable divider (40, 240) sealing the first fluid chamber from the second fluid chamber; characterized in that the hydraulic assembly comprises a bag formed by a first flexible layer, a second flexible layer and a third flexible layer, wherein perimeter sealing of the first and second flexible layers forms the first fluid chamber, and wherein perimeter sealing of the second and third flexible layers forms the second fluid chamber, and wherein the second flexible layer is the flexible movable divider; the tubing is couplable to the hydraulic assembly at an inlet connector of the bag and the hydraulic assembly is couplable to the extension set at an outlet connector of the bag; the hydraulic assembly comprises a rigid, fixed volume housing inside which the bag is enclosed, the rigid, fixed volume housing having a volume that is substantially identical to a volume of the bag when one of the first chamber or the second chamber is filled to its maximum volume, and wherein the housing further comprises a first relief section that accommodates the inlet connector and a second relief section that accommodates the outlet connector. a drive assembly (5, 205) comprising an intravenous bag (30, 230); and a drive mechanism configured to apply a force to drive fluid from the drive fluid reservoir through a length of tubing (10, 210); and a hydraulic assembly (15, 215) comprising: a first fluid chamber (35, 235) having an inlet port configured to be in fluid communication with the drive fluid reservoir through the length of tubing; a second fluid chamber (45, 245) having an outlet port configured to be in fluid communication with a patient extension set (20, 220); a flexible, movable divider (40, 240) sealing the first fluid chamber from the second fluid chamber; characterized in that the hydraulic assembly comprises a bag formed by a first flexible layer, a second flexible layer and a third flexible layer, wherein perimeter sealing of the first and second flexible layers forms the first fluid chamber, and wherein perimeter sealing of the second and third flexible layers forms the second fluid chamber, and wherein the second flexible layer is the flexible movable divider; the tubing is couplable to the hydraulic assembly at an inlet connector of the bag and the hydraulic assembly is couplable to the extension set at an outlet connector of the bag; the hydraulic assembly comprises a rigid, fixed volume housing inside which the bag is enclosed, the rigid, fixed volume housing having a volume that is substantially identical to a volume of the bag when one of the first chamber or the second chamber is filled to its maximum volume, and wherein the housing further comprises a first relief section that accommodates the inlet connector and a second relief section that accommodates the outlet connector. a first fluid chamber (35, 235) having an inlet port configured to be in fluid communication with the drive fluid reservoir through the length of tubing; a second fluid chamber (45, 245) having an outlet port configured to be in fluid communication with a patient extension set (20, 220); a flexible, movable divider (40, 240) sealing the first fluid chamber from the second fluid chamber; characterized in that the hydraulic assembly comprises a bag formed by a first flexible layer, a second flexible layer and a third flexible layer, wherein perimeter sealing of the first and second flexible layers forms the first fluid chamber, and wherein perimeter sealing of the second and third flexible layers forms the second fluid chamber, and wherein the second flexible layer is the flexible movable divider; the tubing is couplable to the hydraulic assembly at an inlet connector of the bag and the hydraulic assembly is couplable to the extension set at an outlet connector of the bag; the hydraulic assembly comprises a rigid, fixed volume housing inside which the bag is enclosed, the rigid, fixed volume housing having a volume that is substantially identical to a volume of the bag when one of the first chamber or the second chamber is filled to its maximum volume, and wherein the housing further comprises a first relief section that accommodates the inlet connector and a second relief section that accommodates the outlet connector.