ok joshua,
i looked and found out that if you put the truck in diagnostic mode and pull back on the lift and tilt hydraulic levers it will display stored fault codes. This may be what you might have done?

Anyhow, fault 20 is described as display showing code 20 and the action light (!) flashing: no lift truck drive operation.

Possible Causes:
- current limit potentiometer P1 out of adjustment
- Excessive drive motor current due to severe ramps, abuse or misapplication.
- Drive pulsing stall timer option set too short for application.
- 80% bypass option ON (option #15).
- current sensor defects:logics defect.

In the 'checks' section is says to:
- Verify current limit setting
- Do problem #16 procedure (this deals with checking current limit sensor and related wiring)
- increase value of option #12 (pulse stall timer)
- disable option #15 if ramp application requires more current than set with P1 current limit potentiometer. (change setting to '0').

Now with all this said yes, if the drive motor brushes are as worn as you say it could be causing a problem in the drive system that is making it see voltages or amp rate readings that will trick it into this code. So yes, change the motor brushes, clean it thoroughly and make sure the sensor wires etc are undamaged. Make sure your directional contactors are in good condition and the contact tips are good too.
Then run the truck and see if anything changes.