Super newbie question:

I am looking for an exploded parts diagram for a Hepburn. I need this to figure out how to lower the hammer tension. Right now, I need two hands to cock the hammer.
Alternatively if someone can explain to me how to lower the tension on cocking the hammer, that would work too.

Thanks,
Paul

That photo is good enough!
Thanks.

Paul

bigpaulespo, excuse me, but from your question it seems you may not have a lot of experience with gunsmithing this rifle in question. Before you do any grinding on the main spring I think you or with someone more experienced help actually determine what the issue is. Reducing the hammer tension can quickly get you to a place where the gun no longer works to fire the primer. Take the gun apart and again if you are not comfortable with that get help. Investigate the issue and then decide a solution. 
This is a rather none specific description of the issue, "Right now, I need two hands to cock the hammer." but if true on the face value then there is something wrong with the rifle. It didn't leave the factory that way when it was built years ago. Don't get yourself to a place where the best description of your rifle will be, "It's been bubba'd"

I have several Hepburn’s and two were Upper Missouri casting kits. One other Hepburn had a broken mainspring that I replaced with one from DZ Arms. Both springs from the castings needed relief and I used a two inch belt sander to remove some of the material as Vall did. I suggested using a file so an inexperienced person wouldn’t take off to much of the spring as with the belt sander one has to be careful. Dz Arms makes an excellent product but I also reliefed that spring too. My other Hepburn’s were originals and didn’t need work as the trigger pull was okay on those. It’s been several years since I did the work but I think DZ Arms might still sell the springs even though Dan has quit making complete rifles. If the OP thinks he has the ability it’s up to him to decide what to do. I’m only making the suggestion as to what can be done, some owners don’t wish to mess with their rifles and that’s fine.
Richard