A hydraulic accumulator consists of a pressure vessel containing two chambers separated by a sealed movable piston or a diaphram. One chamber is pre-charged with nitrogen at aproximately 1/3 to 1/2 of hydraulic system pressure depending upon the specific application. The other chamber is available to be filled with fluid from the hydraulic pressure manifold. In general, hydraulic accumulators store hydraulic fluid under pressure by allowing hydraulic fluid to enter the "wet" chamber through a check valve at system pressure, thereby moving the piston or diaphram and compressing the nitrogen pre-charge in the "dry" side to the system pressure and reducing the volume of the nitrogen, rather like compressing a spring. The hydraulic fluid is prevented from escaping the chamber by the check valve if system pressure drops below the accumulator pressure. In this way, the fluid is trapped under pressure and will maintain this pressure and volume until needed. In the case of hydraulic system failure, including loss of all reservoir fluid, this fluid under pressure is available to be used as a backup source of hydraulic power to the brakes within the limits of the accumulator's capacity. When needed, the fluid is forced out of the chamber by the pressurized nitrogen in the "dry" chamber pushing the piston or diaphram over as fluid flows out of the "wet" side until empty of fluid. As long as the accumulator is servicable and the fluid lines from the accumulator to the brake valves and on to the brakes are intact, emergency wheel braking should be available.
Hope this helps,
Westhawk