I have a 1998 US Market SL500 without ESP. My brake booster is hissing when I depress the brake pedal. I tried replacing the shaft seal (https://www.autohausaz.com/pn/0014300708S), and visually inspected the booster - the seal was shot, so I thought it was my smoking gun. The hissing persists after replacement, but it also was a beast to get it to seat properly and I am affraid it may have been slightly damaged, but not sure.... SO.... I am thinking replace the whole brake booster:

- Part number: 004-430-49-30 - NLA from Mercedes or any parts vendor. Cannot find anything from any vendor as compatible with this year.
- Rebuild kit - are they available, or anyone had luck finding someone to rebuild? I found a service on eBay and they want $1000 to do so - yikes! Obviously they know their worth, but my car is not worth that level of investment.
- Cross compatibility - are they really THIS specific for the 97/98 model years? What is different about the brake booster from the 1999-2002, would it be a problem installing one of those? They look visually the same and have the same braking features for brake assist. Obviously different part numbers though
- Used brake booster on eBay - I can find them sometimes, but they are cost prohibitive and I fear the risk of failure is too high. The only one on there now is $700

I have googled, ChatGPT'd, searched every forum and it all contradicts or is just not a topic of conversation.

Thoughts?