Service brakes only correct? The tandem drive axles already operate off the primary tank. The steer axle service brakes and trailer operate off the secondary. The trailer has its own storage tanks and does not technically operate from the truck storage tank is just fed from it. I would connect a 5/8 line from the secondary tank to the control valve supply port, valve center of crossmember or frame mounted. Tee the control line to the other drive axle service valve control line.