The "secondary safety" as you call it is suppose to be screwed all the way to the back. As far as it will go. I actually never used that safety on my MK-85's. I can't see any good reason for it. Especially since you have to pull the plunger back to fire the rifle.
If you don't have that safety turned all the way back then it could casus a lighter strike. That is possible. Turn it all the way back and set off a half dozen caps. If they all fire then that was probably your problem.
I would tell you what I put on that safety on my MK-85's. But the guys here might frown on the idea. But needless to say I rendered that safety uselss. Tom.