The last top break I worked on had some stuff caked it by the cylinder stop which prevented anything from working right. We can try to diagnose a problem from description only but AR some point it will have to come apart. Sounds to me like a cylinder stop spring after a good look at the diagram. You could try cocking the hammer and while cocked hold the trigger forward, hammer should stay back.
I'm not a professional gunsmith, I love figuring out problems and I buy broken guns and fix them. If you aren't comfortable taking it apart take it to a professional, it is easy to make these old guns into junk if you aren't careful. Stay safe and never assume a gun is unloaded.