Slow Leak in Your Tender?

One possibility is a bad valve. Often, they look horribly sunburned on the outside. However, because the critical parts are inside, completely protected from  UV, most often a good cleaning is all they really need. Fortunately, they are dead simple to service or replace.

1.  Deflate the tender.
2. Grab the inside portion of the valve by squeezing the deflated tube around it. Turn the top counterclockwise using either a valve-specific tool, or a large pair of channel locks or a pipe wrench. They are entirely plastic, so I have never come across one that was stuck. They do not break easily.
3. Remove the top half and give it a good scrubbing with a tooth brush. The sealing surface should be your focus. Lube the sealing surfaces and the outside o-ring very lightly with synthetic or silicon grease.
4. Give the top surface of the inflatable fabric a good scrubbing.
5. Re-install by reversing the process.
6. Give the plug a good scrubbing. Remove the gasket and get both sides. Even if the gasket appears crusty, it's probably just an accumulation of salt and algae. Scrub it up.  If the keeper string is shot, replace it with whipping twine.

You can lubricate the valve in-place with a few drops of glycerin.

Replacement valves are about $15. You will still need to remove the old valve and clean up the fabric.. You can send a picture to Defender Marine if you are not sure, but if the hole in the fabric is about the same size, many are interchangeable. There is probably no need to remove the inside half--it just a threaded holder, never sees sun, and would probably last 50 years. They can be a little tricky to force through old fabric (might tear).

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A parting thought. As I rehab my inflatable for the second time (the first time included paint and a new floor), I find myself amazed by the durability of Hypalon. As the PVC accessories (rub rail and oar locks) crack, the Hypalon remains like new. One one major wear prevention step was the installation of large chafe pads on the inside, where the lifting bridle rubs on the tubes. It has two layers of reinforced rubber, so that when one wears through I can replace it without any risk to the tubes. I wonder if all new dinghies should be fitted with patches everywhere they rub on the davits or transom, much like genoas are fitted with spreader patches by the sailmaker. Of course, this will be a DIY job, since it's hard to say where they should be. Polyurethane caulk works well, as does contact cement--Hypalon cement (a durable, 2-part contact cement) is overkill for holding a wear patch in place.