What is the correct vacuum level to test an oil suction line, according to common practice?

• A. 30 inches
• B. 40 inches
• C. 10 inches
• D. 20 inches

Answer: (D)

Testing an oil suction line with a vacuum is done to ensure the line is airtight and free of moisture or trapped air. You pull a vacuum and then observe whether it holds. Twenty inches of mercury is the standard target because it creates a deep enough vacuum to reveal leaks and moisture without being excessively harsh on the components. If the line is sound, the vacuum stays; if there are leaks, the pressure will begin to rise as air leaks in. Higher vacuums (like thirty or forty inches) are more aggressive than necessary for this check, while a shallow vacuum (like ten inches) may not reliably detect leaks or moisture. So twenty inches of vacuum is the practical, commonly accepted level for testing the oil suction line.