the black drain line from my apec roes-50 keeps popping out of the saddle under the sink.

The tubing isn't fully inserted into the quick-connect fitting on the drain saddle.

You need a firm, straight push. Do not use an insert or any connector on the drain tubing end.

1. Shut off the feed water to the RO system.
2. Pull the black 1/4" drain tubing completely out of the saddle's quick-connect port.
3. Inspect the end of the tubing. Cut it cleanly with scissors or a utility knife if it's damaged or uneven.
4. Push the tubing straight into the quick-connect port on the drain saddle until it seats fully and you feel it click or stop. It should go in about 1/2".
5. Turn the water back on. The constant pressure in the drain line should help hold it in place.

If that didn't work, the quick-connect fitting on the saddle may be damaged. Ensure the drain saddle itself is securely tightened to the drain pipe with its two screws. A loose saddle can wiggle and dislodge the tube.

Never remove the flow control component inside the black drain tubing. It's there to maintain proper drain flow and back-pressure.