If you are cocking the hammer too far towards the full cock position, and then letting it down into what you think is the half cock notch and it keeps going, the problem may be that you cocked it too far. As soon as you hear the click of the sear entering the half cock notch, do not try to cock the hammer further back.
If you do, the fly will just do it's job and prevent the half cock notch from working. That's what it's job is.
If your not making this mistake, read on:
After making sure the gun is unloaded, put it on full cock, and proceed to remove the lock like you described.
Don't force it, just use little taps on the screw until you can start to wiggle the lock while applying pressure to the hammer.
After the lock is out, get your fingers out of the way, grab the hammer spur and gently push up on the sear arm that's sticking out from the lock. Be careful. As soon as the end of the sear arm disengages from the tumbler, the hammer will try to fall.
Lower the hammer gently all the way down.
Looking at the tumbler, it should have two notches in it.
The lowest is the half cock notch. It should look like a pocket for the nose of the sear to go into.
The upper notch should be fairly shallow and it will not have a pocket.
Because your gun has a set trigger, it must have a fly in the tumbler to work properly. The fly is located right at the half cock notch. It is very small and it can swing back and forth (if it's working right).
When you start to cock the hammer, the fly is supposed to swing out of the way so the sear can enter the half cock notch.
If you cock the hammer past the fly, the sear will ride up over it on it's way to the full cock position.
When the gun is fired and the tumbler starts to rotate past the sear nose, the fly gets in the way and blocks off access to the half cock notch. That lets the tumbler and attached hammer tp continue to rotate to the fired positon.
If the fly will not rotate and is blocking access to the half cock notch, try oiling it with a very light weight oil.
As I mentioned, it must swing freely but a thick oil will sometimes cause it to bind (especially in the winter).