We have the Brake Buddy Advantage system. It is activated by inertia when braking or the brake-a-way system, vehicle disconnects from MH and pulls pin activating full brakes on toad.
This unit is powered by 12volts from car or portable battery (booster pack). It is powerful enough not to require any assistance from vehicles power brake system, in fact when setting up, after turning the toads engine off, it pumps the brake pedal 5 or 6 times to ensure that all vacuum power is cleared from system, otherwise brakes would lock up.
I am sure that everyone that responds will give you a different opinion on what they think is best. We bought our system because that is what was available at the time. We had one problem with ours after the warranty expired, but Brake Buddy still repaired it a not cost to us.
Beverley and Ken