express water ro5dx leaking from somewhere under the sink, i see a small puddle. what do i check?

Diagnosis

This issue is typically caused by a failure in the mechanical seal or a "creep" in the quick-connect tubing. Because the RO5DX uses a series of pressurized housings and quick-connect fittings, a leak usually manifests as a slow drip that pools under the sink, making it difficult to identify the exact origin. The most common culprit is an improperly seated 1/4" LLDPE tubing line in a push-fit connector; if the tube is not pushed past the internal O-ring and locked into the collet, water will seep out under the pressure of the storage tank. Another frequent cause is a compromised O-ring in the pre-filter stages. If the housing was over-tightened during a filter change, the O-ring can pinch or deform, leading to a slow leak from the top of the canister. In rare cases, the automatic shut-off valve (ASOV) may develop a hairline crack in the plastic body due to water hammer or excessive pressure spikes from the main line, which causes a steady drip even when the system is not actively producing water. To confirm the source, you must dry all surfaces completely with a microfiber cloth and use a dry paper towel to wipe each joint; the towel will instantly reveal the exact point of moisture.

How to Fix It

Follow these steps in sequence to isolate and resolve the leak:

1. Immediately close the cold water supply valve (the feed water adapter) and close the blue valve on top of the pressurized storage tank to stop the flow of water.
2. Use a towel to dry every single component of the system, including the underside of the housings and the tubing runs.
3. Slowly open the feed water valve. While the system is under pressure, run a dry paper towel around each quick-connect fitting. If the towel dampens, the tubing is not fully seated.
4. To fix a leaking quick-connect, depress the small circular collet (the ring) toward the fitting body and push the 1/4" tubing firmly into the port until it hits the back wall of the fitting. You should feel a distinct "click" or stop.
5. Inspect the three pre-filter housings. If water is dripping from the rim where the canister meets the head, unscrew the housing using the provided plastic wrench. Remove the black rubber O-ring, inspect it for tears or flat spots, apply a small amount of food-grade silicone lubricant, and reseat it.
6. Tighten the housings hand-tight plus a 1/4 turn with the wrench. Do not overtighten, as this can crack the plastic head.
7. Check the drain saddle clamp where the RO system connects to your sink drain pipe. Ensure the clamp is tight and the rubber gasket is creating a vacuum seal against the PVC pipe.

If That Didn't Work

• Inspect the storage tank bladder. If you notice water leaking from the bottom of the tank itself or if the tank feels excessively heavy but the faucet only delivers a trickle, the internal diaphragm may have ruptured. This requires a full replacement of the tank.
• Examine the RO membrane housing (the large horizontal canister). Check the end caps for hairline cracks. If the cap is cracked, the entire membrane housing assembly must be replaced as these cannot be reliably patched.
• Replace the 1/4" tubing sections that enter the fittings. If the tubing was cut at an angle rather than a clean 90-degree perpendicular cut, the internal O-ring in the push-fit connector cannot seal properly. Use a sharp utility knife to trim the tube squarely.
• Check the faucet connection. The small plastic tubing connecting the RO system to the faucet can slip or leak at the compression nut. Tighten the nut by hand and then give it a slight turn with a wrench—do not over-torque or you will snap the plastic nut.

Heads Up

• Never use Teflon tape on the threads of the filter housings; the seal is created by the O-ring, not the threads. Tape can actually prevent the housing from seating deeply enough to seal.
• Always perform a "leak test" for 30 minutes after any filter change before closing the system inside a cabinet.
• If your home has high water pressure (above 80 PSI), consider installing a pressure regulating valve upstream of the RO5DX to prevent future housing cracks.
• Replace your pre-filters every 6 months to prevent pressure buildup that can stress the seals and membranes.