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Fix a backflow preventer that leaks

We'll locate the leak, tighten test cocks, replace O-rings, and tell you when to call a plumber.

Troubleshooting · Home plumbing
15–30 min
What you'll need
  • Replacement O-rings (match your device)
  • Wrench set
Step 1 of 5

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Steps

Goal: Locate the leak, tighten test cocks, and fix or call a plumber.

  • Shut off the water. Locate the leak—test cocks, body, or relief port.
  • Tighten test cock caps. Replace O-rings if worn.
  • Good: No more drip. Bad: Relief port drips constantly—call a plumber.

Tighten test cocks

Goal: Stop leaks at the test cocks.

  • Tighten the caps. If they still leak, replace the O-rings.
  • Good: Leak stopped. Bad: Call a plumber.

When to get help

Call a plumber if:

  • The relief port drips constantly.
  • The body is leaking.
  • Tightening did not stop the leak.

Verification

  • No drips at the test cocks or body.
  • Relief port does not drip when the system is normal.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Shut off water and locate leak Identify where the leak is—test cocks, body, or relief port.
  2. Tighten test cocks Snug the test cock caps; replace O-rings if needed.
  3. Check relief port Flush the system; if relief port drips constantly, internal failure.
  4. Call a plumber Relief port drips, body leaks, or device needs replacement—call a plumber.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Where the leak appears
  • Test cock condition
  • Relief port behavior
  • Steps already tried

Is the leak at a test cock or the relief port?

Test cock leaks are often fixable. Relief port leaks may mean internal failure.

Locate the leak. Test cock: tighten the cap; replace O-ring if needed. Relief port: flush the system; if drip continues, internal failure—call a plumber.

Test cock Relief port

You can change your answer later.

Did tightening or replacing the O-ring fix the leak?

Tighten test cock caps. Replace O-rings if worn.

Tighten the test cock cap. If it still leaks, replace the O-ring. Good: no more drip. Bad: still leaking—call a plumber.

Yes No

You can change your answer later.

Leak is fixed

No more drips. Backflow preventers may need annual testing—check local codes.

Done

Does the relief port drip constantly?

A constant drip from the relief port suggests check valve failure.

Flush the system by opening and closing the downstream valve. Stops: debris may have cleared. Continues: internal failure—call a plumber for repair or replacement.

Yes No

You can change your answer later.

Call a plumber

Call a plumber if: the relief port drips constantly, the body leaks, or tightening did not stop the leak. Backflow preventers protect drinking water—repairs must be done correctly.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a backflow preventer leak?
Common causes: loose test cock caps, worn O-rings, debris holding a check valve open, or a failed internal component. Test cocks are the most common leak point. A constant drip from the relief port suggests internal failure.
Can I fix a backflow preventer leak myself?
You can tighten test cock caps and replace O-rings. Shut off the water first. If the relief port drips constantly or the leak is from the body, call a plumber. Some jurisdictions require certified testers for backflow devices.
When should I call a plumber for a backflow preventer?
Call a plumber if: the relief port drips constantly, the body is leaking, tightening does not stop the leak, or the device needs replacement. Backflow preventers protect drinking water—repairs should be done correctly.

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