fixing a sleeping pad

Assuming you're at home. Inflate and put in the bathtub. Good luck.

The leaks will be more apparent if the bathtub water is soapy. That way, the leaks blow little bubbles. Of course, once you find the leak, then you have to clean and dry it before you try to patch anything.

–B.G.–

"…a very slow leak somewhere…"

I had to climb in the tub, fold the pad, and lean on it to find a small leak.
I found clear water better for seeing the very tiny bubble stream.

With a slow leak around a valve, I could watch the bubble form before it broke free.

or spray soapy water. bubble will form where leak is. Easy to mark exact location.

You can't mark it when it's in bathtub.

Take it to the pool and ply lots of pressure even a micro leak will show itself.