There is a spring and plunger that holds the safety in forward position or safe position after the breechblock is closed,  Clean the crud off around the trigger and safety leave the action standing upside down dribble some Kroil or PB blaster in the space between the safety and action both sides,  Let it sit for a couple of hours add more penetrating oil next morning try moving the safety, breechblock has to be closed for the safety to move back!!
The spring and plunger are in a blind hole this is the only way to lube it without taking the action apart.
LD1 

If you need new plug  srews I have new ones no hand Ken

If you happen to remove the breech block I would first take the firing pin out. Its a real bitch to re-set it and the block won't open should it accidentally release with the block being closed. If you still try to open it, it might break off the end of the firing pin tip. At that point you might now possibly have to fix the firing pin and it would have to be taken out anyway.

I have two Borchardts, a varmint rifle with a hard trigger pull and a military rifle with a surprisingly good trigger pull.

The latter one is prone to slipping off when the breechblock is being pushed out the top, although it does not jar off in use.

What I did was turn a .45 caliber brass piece, with the 10-32 or whatever screw shank threaded in so it would go on the end of my cleaning rod, and push it down the barrel against the protruding firing pin.

Holding this pressure, I could either push the block back down against the cocking cams so it would catch for another try, or continue pushing the block up and out.  The top of the barrel kept the firing pin back until the block came out.  It didn’t snag or make a line on top of the breech.

The varmint rifle has a gunsmith-made plug atop the rear of the block.  This can be removed and a screwdriver inserted to lever the firing pin back if it slipped forward.  This was, apparently, a not-uncommon modification for 78 models modernized for small caliber smokeless cartridges.  The big end on the cleaning rod would not work in these calibers.

Then, as now, disassembling and reassembling Borchardts was not for the faint of heart.  One can sympathize with that letter from John Lower (IIRC) to the Sharps factory asking them to take their New Models back in exchange for the 74 version.

It wasn’t because his unsophisticated customers wanted that big outside hammer for looks, it was because it was easy to take the 74 model apart for cleaning and maintenance out at the hunting camp.  If you contemplate taking the 78 apart, you might well need a machine shop to do so.