If the cabinet under your kitchen sink is damp after you change a filter in St. Louis, the leak is usually at one of a few seals or fittings. Under-sink filter housings are simple canisters with a handful of joints, and most leaks come from a seal that was not seated properly, was pinched, or has hardened with age. This guide gives you a practical order for finding the source before you buy anything new.
Start by drying everything
You need a clean starting point before you can tell where water is coming from. Lay a dry towel or paper towels under the housing, wipe the bowl, the cap, and the inlet and outlet fittings, and close the cabinet door. Check again after a short time. Water that shows up at a joint marks where it is escaping.
If the cabinet floor is wet but the housing looks dry, water may have run down a supply line from higher up. Look at the valve and the tubing above the housing too.
Check how the bowl is seated
Most housings use a bowl that threads or twists onto a head, with a large round gasket between them. If the bowl was removed for a filter change and not fully tightened, water can seep from the seam whenever the system is under pressure. Confirm the cartridge is seated, confirm the gasket is sitting in its groove, and then tighten the bowl by hand until it stops. Do not use a wrench on plastic bowls, because cracked bowls are a common result.
Inspect the O-ring
The O-ring is the most common cause of a small drip. Remove it from its groove and look for flattening, cracks, grit, or a hair caught in the curve. Wipe it with a dry cloth and check the groove for debris. A healthy O-ring is flexible and has no flat spot.
If the ring looks dried out or has a visible nick, replace it with one that matches the size on the old ring or in the housing manual. Use only a lubricant the manufacturer allows, because some rubber materials swell when exposed to oil-based products.
Look at the push-to-connect fittings
Many leaks start where the supply tubing meets the housing inlet and outlet. Confirm each tube is pushed fully in, cut square, and not scratched where the fitting grips it. Pull gently on each tube to confirm it is locked. If a tube has a crease or a ragged end, cut a short piece off square and push it back in past the grip ring. Also check the release collar on each fitting. A cracked collar can let a tube slip under pressure.
Check the mounting bracket
A full housing can flex its bracket and pull on fittings over time. Make sure the bracket is screwed firmly into the cabinet wall and that the housing is not hanging from its tubing. Look at the cabinet floor as well. A soft or warped panel means water has been sitting there for a while, which is worth fixing on its own.
Test one change at a time
Make a single change and then test it. Turn the supply back on slowly, watch the joints for several minutes, and check the cabinet floor again. If you replaced the O-ring, do not also re-seat the tubing in the same step, because then you will not know which fix worked.
When to stop and get help
- The bowl or head is cracked, which means the housing needs replacing.
- The leak comes from the supply valve under the sink rather than the filter housing.
- Water keeps coming out after every seal and fitting checks out.
For the last two cases, close the supply valve under the sink first. A typical St. Louis kitchen has its shutoff under the sink, usually turned clockwise to close. If the valve will not turn or does not stop the flow, a licensed plumber should take a look. This guide covers only the filter housing and its direct connections, not building-wide pressure or well water systems.