Air reservoirs are pressure vessels storing compressed air for use in auxiliary vehicle systems. Predominantly found in vehicles with air brake systems, they provide a readily available air supply to maintain braking force and operate pneumatic components. These reservoirs are constructed from high-strength steel or aluminum to withstand sustained high pressures, typically ranging from 100-120 PSI. Integral safety valves prevent over-pressurization, while drain valves facilitate moisture removal. Capacity is specified in gallons or liters and varies based on vehicle application and system requirements.