I'm not sure exactly what you are trying to do, however, I have a multitude of theses packs and use them for various devices so I can tell you this:
You cannot attach the jumper cables to a 12v battery AND use the USB or 12v output ports at the same time.
When using the jumper cables, they are not meant to run continuously - they just provide a starting current and then stop.
When using the output ports (USB or 12v), the pack will stay running as long as the connected device is pulling power from it and there is a charge left in the pack. Once the device is turned off or otherwise stops pulling power, the pack will shut down automatically.
If you are trying to power a brake system that lays dormant until you engage the brakes, then no, these packs won't work. You would need a different type of pack that does not have an automatic shut off.