1. Remove the mainspring. This takes the tension off the tumbler.
2. Remove the sear spring. Its held on with one screw, and then some kind of notch or hole in the lock plate.
3. Replace the sear spring with the new sear spring.
4. Replace the mainspring.
You are done.
Check Bob Spenser's Black Powder Notebook for Percussion lock terminology and pictures of the backside of most percussion locks. Some have coil springs instead of flat, or "V" springs.
Coil spring will usually have a strut running up the middle of them, and, when the hammer is cocked, a hole in the strut large enough to put a small nail, or paper clip is exposed. You put the wire or clip through the hole, and lower the hammer.
This compresses and holds the coil spring to the strut so that the entire unit( spring and strut) are lifted out of the lock. You want to use the lock's own leverage to compress any spring, before removing it, as that takes that spring's tension off the parts it affects. Depending on how its constructed, you may receive simply a new coil spring, which will require you to remove the existing spring from the strut. Clamp the strut in a bench vise, or even vise grips, and pull the pin from the hole. Then slowly release the strut from the vise, holding onto the spring and strut. ( Put a rag over the palm of your hand so you don't stab yourself.)
To compress the new spring on to the strut, hold the strut in your vise, leaving room for access to that hole. Now use needle-nose pliers, or some other form of tool( you can drill a hole in a piece of steel or aluminum, or thick brass, and run that down on the strut, to use to compress the spring. When its compressed, slip your wire into the hole to hold it. Now you can replace the strut and coil spring in the lock.
I don't know what your particular CVA lock spring look like. There were several different designs used by the company. I have two of them. One uses a coil spring for the mainspring, but the other doesn't. I didn't find a coil spring being use as a sear spring in either gun I have. But, that doesn't mean its an impossible problem to solve.
Locks are fairly straight forward machines. The pictures in Spenser's site should give you an idea of the relationships of the various parts to each other.Working the lock while watching the backside of the lock to see what moves, where, and when will quickly teach you how the lock is designed to work. :hmm: :thumbsup: