hard steering at low speed
 

When parking or turning, steering becomes very hard. A few months ago a new power steering pump was put on. Belt seems tight enough. If I am at a low speed turning sharply and I give it GAS, steering becomes lot easier. At idle speed when turning, steering becomes VERY hard. Power steering fluid is OK.
1992 chevy truck 1500 cheyenne 6 cyl.

Any suggestions on what or how to solve problem?

TIA- Steve45