sorry for "me writing late" - I must have overlooked Your answer.
I doubt (Your) "(activated) Add-Player-Unit-Relay pulls the "Trough-Switch" down".
To me the order / sequence is: Ball rolls into the Outhole (below the flippers) -> -> -> Ball is kicked towards the "Launching Lane" -> Ball rolls over the "Through-Switch" (that switch closes for a short period of time) -> the "P-Add-Player-Unit-Relay" starts pulling (Score-Motor runs) -> sometimes later a switch on Score-Motor opens and by that the "now-open Score-Motor-Switch" cuts current to the "P-Relay (it stops pulling)". If You do not agree on this: Please write.
I do not have a "Bronco pin / schema" - I look up in my "Surf Champ (pin / schema)". It would be helpful, if You would show a PDF showing the section (I am interested in) of the Bronco-schema.
Look in Your schema -> "P-Relay". Nearby is the "Through-Switch" drawn. Also nearby there is a switch drawn (open) called "P". The ball rolls over the "Through-Switch" -> "P-RELAY" gets current -> "P-RELAY" start pulling - and by pulling: "P-RELAY" closes its "P" switch - the "Self-Hold Circuitry (of P-Relay) is established.
Look in Your schema for the coil "Add Player Unit" (there are MANY switches nearby). I would like to see a section of the schema showing the "Add Player Unit" and these (many) switches involved.
I describe here "situation in my Surf Champ":
A long wire Transformer-RED -> Fuse -> Switches -> -> "F-Extraball-Relay-Switch*****" -> "U-First-Ball-Relay-Switch*****" -> "Motor-2C-Switch*****" -> "P-Switch*****" -> "Switch-on-BX-Last-Ball-Relay*****" -> "Score-Motor-Switch-1A*****" -> Z1-1st-and-2nd-Player-Reset-Relay*****" -> Z2-3rd-and-4th-Player-Reset-Relay*****" -> "Add-Player-Unit-COIL".
When only one Player is playing: This "Add-Player-Unit-COIL" should be pulsed 4 times (by the "Score-Motor-Switch-1A" and so the Player-Unit is stepped up to the next ball of Player-1.
"Score-Motor-Switch-1A" must pulse (Open -> Close -> Open -> Close etc.).
ALL other Switches***** MUST BE CLOSED ! There are a lot of switches for You to check.
Example: "F-Extraball-Relay-Switch*****" - If the Player has gained an "Extraball": The Player shall get a Ball "NOT stepping up to the next Ball". IF YOUR "F-Extraball-Relay" is "faulty-pulling", then its switch is open -> NO step-up of Player-Unit. IF YOUR "F-Extraball-Relay is (correct) not pulling, BUT the switch is "faulty-open" (switch-gap or dirty-contacts or Wire-broken-off-the-solder-lug) -> NO step-up of Player-Unit.
IF YOUR "U-First-Ball-RELAY" has (faulty) NOT fallen: Its switch is open -> NO Step-Up ... If Your "U-First-Ball-RELAY" has fallen - but its switch is "faulty-open (switch-gap / dirt / wire-broken-off)" -> NO Step-Up.
The same thing on ALL OTHER (Relays) Switches ...
Bill, You have a lot of (Relays) Switches to check ... Do You have a "Jumper-Cable with Gator-Clips" we could use for testing ? Greetings Rolf.
P.S.: A PDF of the section of the schema (I am interested in) would be very helpful.