Keep tightening until the leaking stops.

It normally takes a pair of wrenches, and a pretty decent amount of force.

Brass compression fittings are preferred over anything with a seal.

Seals dry out with age and start to leak.

Dont forget to use the compression insert fittings when using plastic tubing

"I know of no "compression insert fittings" unless that is the brass part that slips over the tube."

It is a small brass nipple that slides inside the plastic tubing.

Using copper tubing would be a better solution.

The unit has to slide back with no access so I don't think copper is an option. It is 22 inches deep installed in a cabinet with walls on the side so I'd have to cut through the back wall to access it. Not impossible since it is the garage but that does seem extreme.

When you use a brass compression ring on plastic tubing, it will leak if you don't have the insert used for plastic. Go to HomeDepot and buy a package of these. It will also come with a plastic compression ring which is preferable in this situation.

Watts A-8 96760-PT Brass Insert and Delrin Sleeve.

Thanks weed - I'll check it out later today.

"The unit has to slide back with no access so I don't think copper is an option."

You put a loop or two of copper tubing as large as you can fit behind the unit.

Pull the loops out like a large spring to connect up (do NOT straighten the tubing) or pull any further than you need.

After making the final connection yo simply push the unit back in.

The coils of copper will collapse back down between the unit and the wall and not be any thicker than the plastic tubing you are trying to use.

brickeye - thanks for the advice. The length limitation probably makes the coiling hard to do. Maybe with experience I could do it but there just isn't enough extra length to have large enough coils for me to be comfortable.