Suspecting a leak in your cistern is stressful — especially when it's your primary water supply. The good news: with a methodical approach, you can usually pinpoint how to find a leak in a cistern without calling in a contractor. Here's what we've learned from years of installations and service calls.
Step 1: Confirm the Leak Is Actually in the Cistern
Before assuming the cistern is the problem, rule out your plumbing. With no water running inside the house, watch your pressure gauge on the pressure tank. Is it dropping?
- If yes — close the whole-house shut-off valve (usually right after the pressure tank). This isolates the system between the cistern and the valve. If pressure is still dropping, the leak is in that section — most likely a faulty foot valve or a compromised water line inside the cistern. If pressure holds, the leak is downstream — often a running toilet.
- If no — the pressure system is intact. Move on to Step 2 to check the cistern tank itself.
Step 2: Measure the Water Level Overnight
Before bed, shut off power to your pump and take a precise measurement of the water level inside the cistern. Write it down. In the morning, measure again. If the level dropped with no water used, you have a cistern leak. Now it's time to find exactly where.
Step 3: Locate the Leak — Three Methods
The Waiting Game
Stop using water for several days and monitor where the level stabilizes. The point at which it stops dropping is the approximate elevation of your leak. Simple, but slow — and not practical if this is your primary water source.
Food Coloring Injection
With water still in the tank, get inside and use a syringe to inject brightly colored food dye along the perimeter walls. With a flashlight and patience, watch for the dye to get drawn toward a wall. If it pulls into the concrete, that's your leak point. This works best in concrete block cisterns where voids are common at mortar joints.
Pump Down (Most Effective)
Our preferred method. Pump the tank completely dry and get inside while it's draining. Once empty, water that was previously flowing out through the leak will reverse direction and flow back in — because the empty tank is now the path of least resistance. You'll see steady drips or streams at the exact failure points. The downside: you lose your stored water. But the diagnostic accuracy is hard to beat.
How to Fix a Leaking Cistern
Once you've found the leak, you have a few options depending on severity:
- Hydraulic cement patch — for isolated cracks or joint failures. Apply directly to the wet surface; it sets fast and bonds well to concrete.
- MasterSeal 581 full re-coat — the right call for older tanks with widespread seepage. This potable-grade waterproofing cement is applied to the entire interior surface. Always ensure proper ventilation and respiratory protection when working inside a cistern.
When in doubt, re-coat the tank. A full interior application of MasterSeal 581 is a long-lasting fix that removes the guesswork. Questions about your specific situation? Call or text our team at (937) 949-1100 — we're happy to help you diagnose and spec the right repair.
