How many inches of vacuum should be used to test an oil suction line?

• A. 5 inches
• B. 10 inches
• C. 20 inches
• D. 50 inches

Answer: (C)

Testing the oil suction line with vacuum works by pulling the line down to a vacuum and watching whether that vacuum holds. If the line has leaks, air will enter and the vacuum will fall, signaling a leak. Twenty inches of mercury is used because it provides a strong, easily read vacuum that most service pumps can achieve and maintain, making leaks easier to detect. It’s deep enough to reveal small leaks that lighter vacuums might miss, but not so extreme as to cause unnecessary issues or require special equipment. If the line holds vacuum at this level for the test period, the line is considered leak-free; if not, leaks should be repaired before charging the system.