What has me confused is that you seem to be trying to operate with the GW-180 transformer at the same time you are trying to use the Brick/Powermaster. How do you have these two power sources wired together to your track????
I would disconnect the GW-180. Put the Powermaster in Command mode. Power on brick - there will be no power to the track. To run a conventional engine, you use the red Cab1L knob to turn up or down the voltage to the track. For a TMCC/Legacy engine, simply turn the red knob up to get full voltage out of the powermaster, then address the engine, and control as normal.
I think that however you have the GW-180 wired to the Brick/Powermaster combination is killing the Legacy signal return path in the track (TMCC /Legacy signal is transmitted through the air via earth ground in your house wiring, and the return path is via the track)