Power steering is heavy at low speed/idle

This is an issue on my b-body, but surely the same principles apply so I'm asking for help here.

My steering feels good when I'm cruising down the road, but when I come off the throttle, particularly like when pulling into or backing out of parking spaces, I'm getting little to effectively no power assist. This is no bueno with fat front tires and a big block up front!

So far I've done no trouble shooting other than verifying the pump reservoir is full. Any advice on what I should look for first, and how to resolve this symptom?

Thanks!