If you are new to EM repair, a good starting point is this guide.

http://www.pinrepair.com/em/

I'd start with getting the game to reset and play first, then work on rebuilding sluggish score reels, then tackle non working playfield features or lamps. It's just the order of business that.works for me

Ah yes, sluggish forward movement is one thing. They have to reset though - that's another. Here are a couple of reset sequences from other Gottlieb games around that time period. Not sure which is closer, but maybe you can get some understanding from them.

Mr. Pinball Tip The following information is taken from the January 1975 Gottlieb Out of Sight manual. This may be helpful when debugging a startup problem with this or other Gottlieb 2-player games from the same period.

1. Inserting a coin or pushing the replay button actuates 'S' relay (Start relay).
2. This relay will lock-in through its own switch and a motor 2B switch.
3. 'S' relay starts the motor running.
4. 'SB2' and the total play meter are actuated by a switch on motor 2C through a switch that has been closed on 'S' relay.
5. When 'SB2' relay is tripped, the score units and player unit reset sequence starts. The 1st player score units reset to zero through the P3 switch stack on the player unit by motor switch 1A. When these units reach zero the player unit steps once, the 2nd player score units reset to zero through the P4 switch stack on the player unit by motor switch 1A. When these units reach zero the player unit steps until P5A opens. The score units and player unit are now reset.
6. The control bank is reset by a switch on motor 1B through a switch on 'U' relay.
7. Inserting additional coins or pushing the replay button will actuate the 2nd player relay (PB) through motor 2C switch.
8. Place the ball in the out hole. The ball return switch closes and completes the circuit to 'O' relay through the zero position switch on the bonus unit and 'QB'. 'O' relay locks-in through its own switch and a switch on motor 2B. When 'O' relay is energized, motor 4C actuates the ball return coil (which kicks the ball onto the runway) through a switch on 'O' and normally closed switches on 'P' and 'XB' relays. The ball is now on the runway and is ready to be put into play.
9. The remaining balls that enter the outhole are kicked across the trough switch which pulls in 'P' relay. 'P' relay pulls in 'O' relay which runs the motor. Switches on 'XB', 'ZB', motor 2C, 'P', and motor 1A advances the 'Player' unit the required number of steps determined by the 'Player' unit switches and the player relay. (PB)

Mr. Pinball Tip:
The following information is taken from the August 1974 Gottlieb Magnotron manual. This may be helpful when debugging a startup problem with this or other Gottlieb 4-player games from the same period.

1. Inserting a coin or pushing the replay button actuates 'S' relay (Start relay).
2. This relay will lock-in through its own switch and a motor 2B switch.
3. 'S' relay starts the motor running.
4. 'SB' relay and the total play meter are actuated by switches on motor 1A and motor 4B through a switch that has been closed on 'S' relay.
5. When 'SB' relay is tripped, the reset relays 'Z1' and 'Z2' are energized in sequence to reset the score units and the 'player' unit. 'Z2' opens the circuit to 'Z1' until all the 3rd and 4th player score units are reset through motor switches. When these units are at zero 'Z2' falls out and through a normally closed switch on 'Z2', 'Z1' is energized. 'Z1' stays energized until all the 1st and 2nd player score units are reset through motor switches. At the same time the 'player' unit steps up to its zero position through a motor 1A switch in series with a switch on 'SB' and a run-off switch on the 'player' unit (P5B).
6. The control bank is reset by a switch on motor 4C through a switch on 'U' relay.
7. Inserting additional coins or pushing the replay button will actuate the 2nd player relay (PB2) through motor 4B switch; the 3rd player relay (PB3) through motor 2C and PB2 switches; and the 4th player relay (PB4) through motor 1D and PB3 switches; in that order.
8. Place the ball in the out hole. The ball return switch closes and completes the circuit to 'O' relay through the zero position switch on the bonus unit and 'QB'. 'O' relay locks-in through its own switch and a switch on motor 2B. When 'O' relay is energized, motor 4C actuates the ball return coil (which kicks the ball onto the runway) through a switch on 'O' and a normally closed switch on 'XB'. The ball is now on the runway and is ready to be put into play.
9. The remaining balls that enter the outhole are kicked across the trough switch which pulls in 'P' relay. 'P' relay runs the motor. Switches on 'XB', 'ZB', motor 2C, 'P', and motor 1A advances the 'Player' unit the required number of steps determined by the 'Player' unit switches and the player relays (PB2, PB3, PB4).

What do you mean by "the score reels do not reset properly"? To help, we need a better description of what is happening:

Do they continuously spin? Stop at random or specific numbers? Do the player 1 reels stop and the player 2 reels start? Is the score motor continuously running until you switch off the machine? Is the ball ejected into the shooter lane?

I'd start at the beginning, why isn't the start relay activating when there are credits on the game and the replay button (red door switch) is pressed?

Can you test this circuit with alligator clip (or EZ-hook) jumper wires?

http://www.pinrepair.com/em/index3.htm#features
https://pinside.com/pinball/forum/topic/using-schematics-and-8-foot-jumper-wires

Unfortunately, I don't have any Big Brave schematics, but I have Duotron's (a 2 player from the same year). Locate these circuits on your schematic, it should be similar (maybe even identical), wire colors are probably different.

Untitled 2 (resized).jpg

Obviously there could be more wrong than this. If it were me, and I know this is supposed to rotate and "attempts" to rotate and doesn't get there, then the switches will not be in the proper position for the rest of the sequence. I'd probably pull this reel and at least clean it enough to get it operating, then examine the sequence again.

Note this line in the sequence:
"When these units reach zero the player unit steps once, the 2nd player score units reset to zero"
If these 1st player score reel units are not getting to zero, the player unit won't adavance, and the 2nd player score unit will not start it's reset sequence to zero.

How far does the motor rotate before it stops? One third of a turn, a full turn, multiple full turns? The start relay (S) starts the motor turning but SB2 has to activate to keep the motor turning through the reset cycle. Score reels are not going to cycle if the motor is turning off early.