power steering not working pumps fine

hiya av got a defender 90 qwith a 200tdi dropped init, atm ii have no power steering an its not the pump thats broke the pump is fine as its a new 1, theres nothing coming out of the bleed nipple on top of the steering box wen u slacken it off. just wondering weather one of the feed pipes or return pipes is blocked if possible but i drt see how that cud be, or is it the steering box thats the problem? any help will be much appricated cheers