The volume of fluid in hydraulic reservoirs changes due to heat and system operations. As the hydraulic fluid level rises and falls, it can cause turbulence, condensation, air/oil emulsification, and contamination. All pose potential problems for hydraulic components in the circuit. As a solution, Pronal, a French company, invented a separator an inflatable bladder made from heavy-duty synthetic fiber, coated and vulcanized inside and out with an oil-resistant elastomer. The separator inserts in the reservoir and inflates as the dropping fluid level creates a vacuum, expanding to occupy any voids. When fluid returns to the reservoir, it displaces the air and deflates the separator. If the reservoir is cluttered with pipes and nozzles, the separator can be placed outside the reservoir using flexible or rigid piping. The device can also be configured to let the reservoir work underwater.