It finally got warm enough here in the Eastern Sierra to work under my
1982 123 TDT. I took the leveler valve off last week and took it apart.
I should have photographed it every step along the way, because you end up with a few possibilities when you put it back together that seem equally likely.
I did not find anything that looked like a separate check valve inside. And there appeared to be wear on the parts that would be allowing a slow leak back of the fluid, i.e., sagging overnight.
So I ordered a used valve from a wrecking yard. It hasn't been delivered yet, so I can't report on whether or not that solved the problem. The yard I used guarantees the valve and will send another one if the one they send does not work right.
I will keep the list posted.