Dry the system completely, place paper towels under each connection, restore pressure and watch for the first wet point. Then shut off the feed before disassembly.
Push-connect fittings and tubing ends
A tube that is not inserted fully, is cut at an angle, or has a scored end can leak at a push-connect fitting. Recutting tubing squarely and reseating it may solve the problem when the fitting itself is undamaged.
Do not keep pushing damaged tubing deeper into a fitting; replace compromised tubing or hardware.
Filter housings and O-rings
Traditional canister systems can leak when an O-ring is twisted, dirty, dry, damaged or missing. Cross-threading or overtightening a housing can also create leaks instead of preventing them.
Depressurize the system before opening a housing and inspect both the sealing surface and the O-ring.
Faucet, tank valve and drain saddle
A leak near the faucet may come from the faucet body, supply tube or countertop connection. Near the storage tank, distinguish a tank-valve leak from condensation or tubing leakage.
At the drain, confirm the saddle and tubing are secure and that drain water is not backing up from a plumbing problem.
When not to keep repairing the same unit
A single seal or fitting is normally a small repair. Repeated leaks from brittle housings, aging tubing, multiple valves or an old tank can be a sign that the system is reaching the point where replacement is more sensible than serial repairs.