Quick Answer:
A small leak at the test port fitting is often caused by a loose or worn seal. Start by shutting off the water and gently tightening the test fitting. If that doesn’t stop the leak, replace the O-ring or packing, then retest the device at low pressure before returning it to service.
Why This Happens
Common causes for a leak at a test port include a loose test cock fitting, an aged or cracked O-ring, or dirt and corrosion that prevent a proper seal. Freeze damage or recent testing work can also disturb the fitting or seal. If the valve body itself is damaged or the seat is compromised, the test port may continue to leak even after the fitting is serviced. For related issues, see Backflow preventer won’t reset and Backflow preventer dripping constantly.
Step-by-Step What to Do
1. Gather what you’ll need
- Adjustable wrench or correct-size wrench for the test cock cap
- Replacement O-ring or test cock washer (match the size and material)
- Light lubricant (plumber’s silicone grease) — don’t use petroleum oil on rubber
- Rags, bucket, and safety goggles
2. Isolate and depressurize
- Close the appropriate shutoff valves to isolate the backflow assembly from system pressure.
- Open a downstream faucet to relieve residual pressure and drain water from the line.
3. Inspect and tighten gently
- Clean around the test port so debris won’t fall into the opening.
- Use a wrench to turn the test fitting just enough to snug it—do not force it. This is the “tighten gently” check; sometimes a loose fitting is the only problem.
- After tightening, slowly re-pressurize the section to check for leaks.
4. Remove the cap and check the seal
- If tightening did not fix it, remove the test fitting cap and inspect the O-ring or washer for cuts, flattening, or hardening.
- Clean the seating surfaces; small particles can keep a seal from seating properly.
5. Replace the O-ring
- Fit a matching O-ring made from the correct material (usually EPDM for potable water) and lightly coat with plumber’s silicone grease.
- Reinstall the cap and hand-start the threads to avoid cross-threading, then snug up with a wrench.
6. Retest
- Slowly reopen the isolation valve to bring the assembly back to pressure.
- Watch the test port while pressure increases. If the leak is gone, monitor for a while to ensure the repair holds.
- If the port still leaks after replacing the O-ring and proper tightening, the internal test cock or body thread may be damaged and will need further repair or replacement.
What Not to Do
- Do not overtighten brass threads — they can strip, crack, or gall, making the leak worse.
- Do not use pipe dope or thread sealant on test port O-rings; they should seal against a smooth seat with an O-ring or washer, not with sealant.
- Do not try to force a damaged cap back into place—forcing can hide a larger failure or cause internal damage.
When to Call a Professional
- If the leak continues after replacing the O-ring and careful tightening.
- If threads are visibly damaged, stripped, or cracked.
- If the assembly is part of a complex commercial system or a health-critical supply where testing and certification are required.
Safety Notes
- Always depressurize the section before loosening any fittings. Unexpected pressure can spray water and cause injury.
- Wear eye protection when working on pressurized plumbing.
- If you’re unsure which valves control the area, stop and get help rather than guessing—accidentally shutting the wrong valve can affect neighbors or critical equipment.
Common Homeowner Questions
- Can I fix this myself? — Often yes for a loose cap or a worn O-ring, but stop and call a pro if you see damaged threads or continue to leak after replacing the O-ring.
- How do I pick the right O-ring? — Match the size and material to the original; bring the old O-ring to a supplier or check the device model for specs.
- Will tightening void a warranty or inspection? — Minor tightening usually won’t, but avoid excessive force and check any local testing or certification rules if the device is inspected regularly.