Here are two examples of the Douglass Pressurized Reservoir. In (A), a low pressure is created by the Venturi action of flowing oil. This would cause air to come in through pipe (1) to relieve the low pressure; and a pressure head would be formed. The relief valve on the vent at the top of the tank would regulate the pressure. In (B), the spring load and piston keep oil under constant pressure. This type of design is bad because you couldn’t fill the reservoir with oil easily.

Boot Strap (self-pressurizing) Reservoirs

Boot Strap reservoirs are used in spacecraft and are used to maintain positive pressure in the hydraulic lines. Actually, there is some return line back pressure since P₂ is greater than the return line pressure. For example, if the pressure line is at 500 psi and it acts on the 1 square inch piston surface (see figure below), the force generated would be 500 lb. Since the boot strap system incorporates piston 1 and piston 2 into a combined piston (see Pascal's Theory --Section 1.2), this force would be converted into a pressure, P₂, of 5 psi acting on the 100 square inch surface of the piston, A₂ .

This type of reservoir is very difficult to maintain. Also, bubbles trapped within this system cannot be removed very easily. Its good points are that it is foamless and has no air which can be trapped in the fluid due to its operation.