Follow this link to see a post with pictures that define the various lock parts.
If your computer savy you might even want to do a "screen grab" so you will end up with a photo you can store in your computer.
http://www.muzzleloadingforum.com/fusionbb/showtopic.php?tid/254598/
These locks are fairly simple and if you bare with me I'll try to explain how they work.
There is the mainspring that provides the power to move the hammer.
Most locks use a flat spring for this but the TC and Lyman locks use a coil spring. Either way, the mainspring pushes on something called a tumbler.
The tumbler rotates and because the hammer is attached to it on the outside of the lock, as it turns, the hammer moves.
The tumbler on your gun has two notches cut into the round part of it that is inside the lockplate.
Often there is a 'bridge' that is held against the inner surface of the lockplate with a hollowed out area that covers much of the tumbler.
This gives support to the tumbler to help it withstand the heavy spring pressure produced by the mainspring.
This 'bridge' is called the "bridle".
There is a lever with a small nose on it called the sear. The nose of the sear engage these notches in the tumbler. The sear has a light weight spring that pushes its nose against the body of the tumbler.
The notches are the half cock notch and the full cock notch.
The half cock notch is a hooked affair that traps the nose of the sear to keep it from accidentally disengaging.
The full cock notch is just a small shelf that the nose of the sear engages to hold the tumbler and hammer in the full cocked position.
A slight movement of the sear will slide its nose off of this full cock notch allowing the tumbler to turn and the hammer to fall.
The sear has a arm or projection that sticks out away from the inner surface of the lockplate.
When the trigger blade pushes up on this arm the sear is moved to release the tumbler.
Your gun has a "set trigger" which has two blades that can hit the sear arm.
The front triggers blade simply moves upward with you pull the front trigger.
The rear triggers blade normally doesn't touch the sear arm unless the trigger is "set" and released.
Setting the rear trigger cocks it by compressing a heavy spring. Once it has moved to the "set" position a small latch on the front trigger engages it to hold it there.
A very light movement of the front trigger will release this latch.
When it does, the heavy "set trigger" spring drives the set trigger forward rapidly.
As the set trigger moves forward, the blade on top of it rapidly moves towards the sear arm.
Its high speed gives it the momentum needed to hit the sear arm causing it to release from the full cock notch in the tumbler. The hammer starts to fall.
Because the momentum of the rear set trigger is now absorbed by the sear arm, it bounces away from the sear arm and returns to its unset position.
Meanwhile, the tumbler is moving and the hammer is falling and the sear spring has pushed the nose of the sear back against the outside of the tumbler.
As it turns, the half cock notch is rapidly approaching the nose of the sear.
Without something to prevent it, the nose of the sear will go back into the half cock notch stopping the hammer fall and possibly breaking the sears nose or the half cock notch.
To prevent this from happening a very small movable part is installed in the tumbler at the half cock notch. It's one and only job is to keep the nose of the sear from entering the half cock notch when the tumbler is moving from the full cock position.
This small part is called the "fly".
The fly is easy to move and it is usually held in place in the tumbler by the bridle.
GEtting back to your guns problem, for some reason, the blade on the rear set trigger is not hitting the sear arm or it is not hitting the sear arm hard enough to cause the nose of the sear from moving out of the full cock notch.
Find out why this is happening and it will be easy to fix.