1985 300D Cali

Hey everyone, I’m a long-time reader but a rare poster. I’ve read many threads on here (and various other forums) about W123 brake booster failure and I’m posting this because I haven’t seen anyone discuss this issue.

I heard that dreaded hiss under my feet when I was chasing vacuum issues. (See below how I determined this is my issue.) I’ve done a lot of research, and I’m almost certain that it’s the poppet seal (rear seal?) on my brake booster p/n 0024304530 that has failed. Yes, I’ve seen people use temporary fixes like zip-tying rubber seals around it, but I want a permanent solution.

TLDR; Has anybody tried one of these URO seals on our brake boosters?

URO 0014300708S Brake Booster Plunger Seal
URO 4836664S Brake Booster Plunger Seal

I’m almost certain it’s my brake booster because...

I removed the main vacuum line from the booster and plugged it. I then hooked up a vacuum tester to one of the two smaller feeds and tested them one at a time. Both smaller feeds held the vacuum around 22Hg. When I plugged the smaller feed (while keeping the tester on the other feed), and reconnected it to the Booster, the vacuum dropped to zero. After I hooked everything back up, I heard the dreaded hissing noise under the brake pedal. The hissing is a steady noise that stops when I apply slight pressure. I don't hear or feel a leak between the MC and Booster. Pretty sure there is no leak since my Booster doesn't have a telltale sign of rust. I will start taking things apart this weekend.