I found this thread:
https://www.askmehelpdesk.com/plumbing/watts-premier-reverse-osmosis-tank-wont-fill-water-continues-run-318082.html

Similar situation. I've had the RO for over a year, and have changed the filters accordingly. It was working fine (although never had a whole lot of pressure).

It is routed to my refrigerator by the way. Anyhow, my wife got a glass of water, and this thing started gurgling like usual, except louder, and didn't stop until I turned the valve on the top of the tank off, then back on. Once back on, no gurgle.

2 hours later, I got a glass of water, and there it goes again. I looked down the drain, and water is just pumping into it. I had to turn valve off again. When I turned it back on (immediately), the draining/gurgling stopped.

I see no leaks below sink ANYWHERE, and I see no leaks under or behind fridge.

Any suggestions? How do I test pressure of tank (with what tool)?