The first order of business for me on System 6 before I even turn them on is to always:
1. Test the existing fuses both for continuity and value (pin operators tended to do half-asssed fixes on the spot to squeeze out a few more quarters for minimal effort - remember pinballs of that era were built to be "throw aways" - they were meant to make money in their first popular run and then be junked and I add two extra fuses to protect the general illumination at the bridge rectifier for safety,
2 upgrade the power supply with new caps and other parts,
3. lower the high voltage a little to the displays to help preserve them,
4. Test all of the transistors, diodes and coil resistances and inspect all of the coils and switches to see if I can detect any existing faults ((often hinted at by the fuse condition),
5. Fix any battery acid and resolder all edge connectors and pins and replace those with obvious wear / looseness / defect,
6. Move the batteries to a remote holder with a reverse current protection diode,
7. Judiciously plug in and test each circuit board (turning off between connections) in the backbox in sequence so as not to blow up anything and test voltages,
8. Tighten all switch packs,
9. Test all switches lights and solenoids,
10. DO a little play testing to see if I can break anything, or if any coils are weak or switches obviously out,
11. Clean all lights, inserts, and resleeve solenoids, clean them, replace warn parts,
12. Remove (and clean )all plastics and rubbers.
13. Clean and strip playfield and do any playfield touch-ups, replace any targets, broken plastics, etc.
14. Clean underplayfield wire looms, cabinet, etc.
15. Re-clear any touchups. Re-wax playfield.
13. Replace all rubbers and add washer protectors for plastics in judicious spots.
14. Re-test and adjust flippers
15. Replace lights (always replace backglass lights with #47 or LED), or add leds throughout depending on machine and who is having the work done
15. Clean or replace glass.
16. Touch-up and seal backglass if it is flaking.
By this point I am both thoroughly satisfied and bored with the machine, and it purrs like a kitten and either goes back to its original owner (I rarely do them now as it takes so long and I have other employment and hobbies), or goes back into my collection if it's a keeper, or gets sold (although I haven't bought or sold anything in a LONG time).
You need to do your research.
Regarding INSERTS. Insert restoration has been covered a 100 ways to Sunday online in the last 15 years. Plastic / wood shrinks and swells as it's heated and eventually separate and the plastic at the top sinks due to heat and ball wear/weight.
I generally remove, reseat and reglue loose inserts at an even edge level with the playfield. Sometimes I leave it there (cupping at this age is "character") but if it truly affects playability (it has minimal effect), I will fill the cups with CA glue (the more viscous clear crazy glue stuff you can get at RC hobby shops, sometimes I have used water-based polyurethane, but it takes a lot longer to dry) as, if you're careful, it will flow and lay flat. Some people, after regluing and cleaning, and touching up the playfield will have it re-cleared with either a rattle can waterbased clear (that doesn't yellow but takes a month to dry) or automotive epoxy clears.
You can get away with doing only the necessary steps above to get a machine working, but if you aren't thorough on the electronic and mechanics side, you will be continuously fixing the machine AND guaranteed something will break in the middle of your pinball debut party.
