No, no impeller within the helm pump.
Here's a post showing some guy rebuilding his helm
Seastar steering helm pump rebuild/with pics - The Hull Truth - Boating and Fishing Forum
Most likely, a bleed will take care of your problem (in truth, probably a top-off and a little exercising the helm will take care of it, but if you haven't changed the fluid in a while, that's probably a good idea anyway.) If you decide to change the oil, I'd suggest changing the cylinder seals as well, as you'll have to bleed the system after doing so anyway (and they will start leaking - trust me.) The helm seal can be changed without needing to bleed.
Sorry, I know I've seen an exploded diagram of the helm, but can't seem to find it now.