wot cud cause the loss of power steering on 2wd cossie?

it works for a while then stops ive no leaks amd the fluid level is fine and the belt seams tight enough so id say its the pump its self ?

if so is the pump the same as any other sierra ?

cheers

 

ok the problem now fixed,it was the pump it had packed up,incase any1 else has the same problem,

cheers

 

I havent had it myself but a guy that goes to the garage where I work went on friday with the same prob mate