Was been using Promax mechanical disc caliper for a while, 10yrs ago.

1 - Remove the rusty bolts on the brake disc, get them replaced or clean it
2 - Take the pad out and sand them down a bit with sandpaper
3 - Sand the brake disc
4 - Put them back in and tighten the cable ( Replace the cable and housing if needed )
When you done all these, they should work better.
If not, bring the brake pad to bike shop and look for BB5 or BB7 brake pad to see if they are the same shape
If so, you can replace your old brake pad with any BB5 or BB7 brake pad.