A constant pressure system includes a pressure vessel with a vapor-liquid mixture used to provide pressure forces instead of compressed air that is typically used. The vapor-liquid mixture can be a number of substances, such as nitrous oxide, so long as the mixture exists in both the liquid and vapor phases. Importantly, the vapor-liquid mixture must maintain a constant pressure during the dispensing of fluids from the tank, so that the fluids are dispensed at the same constant pressure. As a result, the fluids within the tank can be dispensed at the same pressure as that of the vapor-liquid mixture within the tank, or scaled to a higher or lower pressure value through the use of a pressure-converter valve within the system.