1990 power steering not working

I have a chevy 1990 van 350 & i had a mechanic replace the power steering pump
NAPA forgot to give me the ”reservoir” when it was done (which cost $255!) my steering will sometime work and sometimes not ie … it goes in and out?

What might be the problem?
Could it be?
1) dirt left in as the old resevoir was used? (and I need to flush the fliud?)
2) A faulty NAPA part?

HELP.
THANKS
Scott

Answer: This could just be air in the system. Make sure you bleed the system real good. If that does not help then i would say that either the belt is slipping or the pump is faulty.

A slipping belt could be caused by oil or other fluid on it or a weak tensioner.

A faulty pump could be caused by a bad pressure regulator valve, or maybe the mechanic did not swap the old one into the new pump.