In some cases the fluid gets too hot and the pump quits working. I added a power steering cooler to the Charger after this happened.
I had a worn out 71 Chevy truck that had an intermittent power steering "system" What a TURD ! It would ofter fail at low rpms while making U turns. I changed the pump TWICE and the problem persisted. I think in that case, the steering box itself was AFU.