A bi-level tank includes a transfer tank and a return tank containing a volume of water, including transfer and return components in the transfer and return tanks, respectively, and a transition component. A bellows couples an upper surface of a piston in the transfer tank to the return component that exerts pressure on the upper surface, while a lower surface of the piston is under pressure from a pressured fluid supplied by a source thereof, producing a pressure differential on the piston. Actuation of a force-applying mechanism on the piston sufficient to overcome the pressure differential displaces the piston for exchanging respective volumes of the return component and the fluid from the source. An extensible and retractable constant-volume boot holds the transition component around the bellows and has valves configured to open and close for equalizing pressure between the boot and the transfer tank.