So I checked brake fluid level and it was close to the top (very close)--assuming this is ok. Do I need to bleed brakes now?
I followed the plug back to the trailer and discovered exposed wires (not in rubber sleeve); that the safety chains had rubbed to expose raw wire on green & white. I stripped, wrapped and closed them off from any contact.
Do you think this was causing the pulsing? Surely seems like it might have grounded contact disabling back up and possibly creating surge / jump when braking.