If the rack stays full of fluid, you're probably OK to drive it, assuming that you don't mind flushing out the entire system later. But I wouldn't want pieces of a disentragating pump fouling up my rack.

If air gets into the rack, it's not pleasant at all. I learned this after changing the return hose and not getting all of the air out. The fluid in the rack acts as the (only) steering damper, and it's no fun at all without it.