Quick Answer:

If water is coming only from the vent opening, the most common causes are debris on the vent seat or a worn or damaged diaphragm. Start by relieving pressure, remove the vent cap, clean the vent seat, and inspect the diaphragm. If cleaning and a simple diaphragm check don’t stop the leak, call a pro.

Why This Happens

Backflow vents are small relief points designed to let excess water or air escape. A leak only at the vent opening usually means the vent valve is not sealing. That can happen because:

• Debris, mineral scale, or grit is trapped on the vent seat so the valve cannot close fully.
• The diaphragm or sealing surface is cracked, hardened, or deformed and no longer forms a tight seal.
• Internal parts are worn or shifted, allowing a small continuous path for water.

If you see intermittent or odd behavior elsewhere in the system, it can point to related issues — for example, if the assembly also releases water randomly or is dripping constantly, that helps decide whether the vent leak is isolated or part of a larger failure.

Step-by-Step What to Do

1. Turn off and relieve pressure

• Shut off the supply to the device if possible. Open a downstream faucet to relieve pressure before opening the vent area.
• Wear gloves and eye protection.

2. Remove the vent cap carefully

• Unscrew or unclip the vent cap per the device’s instructions. Keep small parts together so nothing is lost.
• If the cap is stuck, do not force it with metal prybars — use a soft-jawed tool or gentle penetrating spray and let it soak.

3. Clean the vent seat

• Look for grit, gravel, or mineral buildup on the seat and sealing surfaces. Use a soft brush, clean cloth, or a small pick made of plastic to remove debris.
• For mineral scale, a vinegar soak or a mild descaler applied carefully can help. Rinse thoroughly with clean water.
• Reassemble and test briefly — sometimes cleaning the seat fixes a vent-only leak.

4. Inspect the diaphragm

• With the cap off, examine the diaphragm or sealing membrane. Look for cracks, stiffness, distortion, or material breakdown.
• If the diaphragm shows damage or is not seated squarely, it will likely need replacement. Do not try to patch a damaged diaphragm.

5. Reassemble and test

• Put the cap back on correctly and restore supply slowly while watching the vent. If the leak persists, repeat the inspection or stop and call a professional.

What Not to Do

• Do not block vent — never tape, plug, or cover the vent opening. Blocking the vent can defeat the device and create unsafe pressure conditions.
• Do not use sharp metal tools to scrape the seat; you can score the sealing surface and make leaks worse.
• Do not ignore signs of internal damage. Continued operation with a leaking vent can worsen the failure.

When to Call a Professional

• If you cannot stop the leak after cleaning the seat and checking the diaphragm.
• If the diaphragm or internal parts are damaged, missing, or you are unsure how to reassemble the device correctly.
• If local codes require certified testing or repair for backflow assemblies, or if the unit is part of a critical system (irrigation, boiler, commercial supply).

Safety Notes

• Always shut off and depressurize before opening the vent area. Sudden flow can spray water and debris.
• Use gloves and eye protection. Some systems contain chemical residues or dirty water.
• Avoid aggressive chemicals unless you know they are safe for the materials in your device. Rinse thoroughly.
• If you are not comfortable working on the part, stop and call a licensed plumber experienced with backflow devices.

Common Homeowner Questions

• Q: Will a quick cleaning always stop a vent leak?
  A: No. Cleaning often helps, but a damaged diaphragm or worn seat will require replacement or professional repair.
• Q: Can I replace the diaphragm myself?
  A: Sometimes — if you have the correct replacement part and follow the manufacturer’s instructions. If not sure, hire a pro.
• Q: Is it okay to leave the vent leaking while I wait for parts?
  A: Not recommended. A persistent leak can indicate more serious wear and may affect system performance; arrange repair promptly.