I’ve been having trouble with a wheel causing slow flats.

My son rode the bike for a few hours and it was fine. The next morning the tire was flat. This is the second time this has happened. I took the tube off, inflated it and did the usual test with water. There were no leaks. But I was taking any chances, so I took mounted the tire with a new tube on the wheel. I inflated to 50 psi – within ½ hour the pressure had dropped to 40 psi.

I took that tire and tube off the wheel and mounted them both on a different wheel. After 24 hours there are no leaks. Therefore it must be the wheel.

To test the theory that it is the wheel causing the flats, I took a different tire and another brand new tube and mounted them on the troublesome wheel and inflated to 50 psi. After 12 hours the psi dropped to 40.

What could be causing the wheel to make these flats? What kinds of things can I look for in the wheel to locate the issue?

The wheel is a steel 26" one off of an old mountain bike.

Thanks,
Dave