If it won't hold full cock until the trigger is set, the trigger mainspring is overtraveling. The notch it engages in the rear trigger can be relieved some until it doesn't hold the rear trigger bar against the sear, or a stop screw or shim can be added to the trigger plate so the mainspring doesn't push so far down on the rear trigger. The rear trigger bar should actuate the sear bar by inertia only and not put any direct pressure on the sear bar at any time. I would take the lock out and inspect the tumbler for a half-cock notch. I have seen more than one old rifle with set triggers that didn't have a fly or provision for one in the lock, causing the sear nose to become so damaged that it simply would not go in the half cock notch but would still function in the full cock notch.