power steering help

[vinnythespud]

hi, i am wanting to swap my manual steering for power steering, i have managed to get a rack which has three ports in it, does it matter which way i pipe it up, im guessing the big ports are feed and return but what is the small one for. i also need to know what threads the ports are so i can use aluminium fittings and stainless braided hose. any help would be greatly appreciated