looking for guidance, so I have the valve body out and the square Orings are in poor shape. I saw that many people use the harbor freight O-rings...but I assume there is no O-ring from the HF pack to use in lieu of the square Orings?

I saw a thread here somewhere (same thread in multiple sections) that directed me to an eBay kit. Two kits, one with the square and one with orings only. Works a treat.
Recent as well!

I ended up using Orings, others note that the "square" rings are simply spacers

We will see

I'm about to do a flush and valve block cleaning on my car today and have done a bit of research. The square spacer rings appear to have more of a purpose then just spacing. Check out the diagram in post #11 by Knowbenz in this thread.

https://mbworld.org/forums/sl-class-...seal-sl55.html

Replacing square cut rings with o-rings was a problem for me, the o-rings installed in place of the square cut took up more space. When i went to install the valve assembly in the block there was no room for the extra rubber to occupy. When you have the propper material in place the valve should slide into place by hand. Don't force it or use a clamp.... I was able to go down to the local hydraulic repair shop and get the orings as well as the square backup rings.

Hmm....I wonder if I should pull it apart and redo it....I bet ACE harware has the spacers

just use the ebay kit as they are the correct size. You have to be very careful about sizing. There is a couple of threads running around that lists sizing for the square cut rings, but those are incorrect and too large (I know because I bought them and tried). If you get them slightly too large then the square cut rings will shred and send rubber contaminants all through the system leading to more problems. I bought a valve block that was supposedly rebuilt and then failed again. When I took it apart for inspection I found that all of the square cut rings that were put in were sheared off and pieces of them were everywhere b/c the square cut rings were too big.

Repaired it with O-Rings, works fine...but ordering the kit to do another day....repair was not bad

I vote for Ebay kit, used it numerous times with great success.