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RV black tank flush leaking inside: why water shows up under the sink.

One of the nastier surprises in an RV is turning on the black-tank flush and finding clean water leaking from a cabinet. The usual suspect is the vacuum breaker, not the tank itself.

Why the leak is often inside

Many black-tank flush systems route water from the exterior connection up to a vacuum breaker or anti-siphon valve inside the coach, often under a bathroom sink, behind a wall panel or inside a cabinet. From there the line runs down to the tank sprayer. If the vacuum breaker cracks or sticks, water can dump inside the RV only when the flush is running.

Stop the test quickly

  1. Shut off the hose immediately.
  2. Open the cabinet or access panel near the bathroom vanity and wet wall.
  3. Dry the area and look for the highest wet point.
  4. Do not run the flush unattended to “see what happens.”
  5. Check that the black dump valve is open before normal flushing.

Other causes

A cracked flush line, loose fitting, frozen line damage or overfilled tank can also push water where it does not belong. If water appears at the toilet or floor instead of the cabinet, stop and verify the tank is not filling faster than it can drain.

Keep hoses separate

Use a dedicated non-potable hose for tank flushing. Do not use your drinking-water hose on the black-tank flush connection.