Check the fluid. Take the green dipstick cap off and rub the end on a white rag, it should be a red or green colour, if it's brown or black then it needs changing. Also worth seeing if the system needs a quick bleed, start then engine, and with the cap off turn the wheel lock to lock a few times. Could also be the new belt not tight enough. Failing all of that... knackered PAS pump or a leak in a PAS pipe