If your under-sink reverse osmosis system uses push-fit fittings, the small plastic tubing connections are one of the first places a slow drip can start. In St. Louis kitchens, where cabinet space is tight and supply lines often run close to the drain and the shutoff valves, a connection that looks seated can still work loose after months of pressure changes and heat under the sink. This guide walks through a simple check you can do with a flashlight, a dry paper towel, and a short amount of time.

What a push-fit collet is

A push-fit fitting holds tubing with a small plastic ring, called a collet, that grips the tube when you push it in. Behind the collet is an O-ring or seal that keeps water inside. Most under-sink RO systems use these fittings on the feed line, the drain line, the storage tank line, and the faucet line. Because the tube is held by grip and friction, it can be partly seated and still look connected.

Shut the system down first

Close the cold supply valve under the sink, then open the RO faucet to relieve pressure. Let the remaining water drain out and check that the storage tank is not pressurized enough to spray when you disconnect a line. If your system has a separate shutoff at the tank, close that as well. Keep a towel under the work area so any residual water stays off the cabinet floor.

Look at each connection

  • Check that the tube sits straight in the fitting and is not bent sharply just past the collet.
  • Make sure the collet lies flat against the fitting body and is not tilted or pushed partly out.
  • Look for a gap between the tube and the collet where the tube seems to slide.
  • Note any discoloration, cracking, or stiffness in the tubing near the fitting, which can appear after long exposure to heat under a sink.

The tug test and the paper towel test

Gently pull each tube straight out with light pressure. A properly seated tube will not slide out. If it moves at all, push it firmly back in until it stops, then test again. Next, dry a paper towel and wrap it loosely around each connection. Turn the cold supply back on slowly, let the system run through a normal cycle, and then check the towel. Any dampness or staining means that connection needs a closer look.

Fixing a loose connection

If a tube pulls out, cut a clean, square end with a tubing cutter or a sharp blade, removing any rough or flattened section. Do not reuse a tube with a cracked or deformed end. Push the tube fully into the fitting until it bottoms out, then give it a gentle tug. Do not overtighten anything, since these fittings rely on the seal and grip rather than force. If the fitting body is cracked or the collet is broken, replace the fitting with the same type rather than trying to patch it.

When to stop and call for help

If you find a leak you cannot stop, if the cabinet floor feels soft, or if the fitting type is unfamiliar, close the main supply and get help before you continue. Many St. Louis homes have under-sink spaces that have been modified over the years, so the tubing may not match what the system manual describes. A licensed plumber can check the lines, and your system manual or the manufacturer's support contact can confirm the correct fitting type.

Keep a simple record

After you check the connections, write down the date and what you looked at. Repeat the same check each time you change filters. A short written note makes it easier to spot a pattern, such as a connection that always seems to loosen after a filter change, which may point to a tube that was cut unevenly or a collet that was not fully seated.