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Getting lines, valves, compressors, pumps, or any other system components to function requires fitting them to one another to form a complete system. Initially the fitting used to form a seal was a tapered thread, which by screwing it in with enough force would create a seal. This was not always completely sealed, and it usually leaked. This type of seal was developed for water and air pressure, and functioned well under a hydraulic system subject to 800 psi to as much as 200 degrees Fahrenheit.