I replaced the master cylinder and brake booster on my '83 300D this summer. It never felt right, always had a pop to it, but the brakes felt better than they ever had so I left it. This week, I started having vacuum issues- no brake boost, locks not working and engine not shutting off.

I disconnected the vacuum hose going to the booster. It reads upper 20's mm on the mityvac with just a little bounce in the needle. When I connect the mityvac directly to the booster, it will not hold any vacuum. Also, with the vacuum line attached to booster the engine struggles to shut down. Disconnected from the booster and plugged, the engine stops immediately.

The booster is bad, right? Am I missing something here. Don't want to pull it all apart for nothing...

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Rich

83 300D
98 E320 Wagon

I have never tried such a test, but my knee jerk reaction is that the mity vac wouldn't make enough volume to operate a brake booster. Can you fashion a test line from the vacuum pump for your test?

Hope this helps.

Sounds like a leaking booster however, I "think" these are like most Euro car boosters where the back of the master cylinder has an o ring on it to seal vacuum. Most US built cars have a seal on the booster and not the MC.

If you unbolt the MC, pull it back and can see deep inside the booster, you need some sort of seal on the back of MC.

Yes, there is a seal on the back of the master cylinder. I guess it's a possibility it has failed. I will check it tomorrow.

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Rich

83 300D
98 E320 Wagon

Did you replace the master cylinder at the same time that you replaced the booster? If you had the old MC on the new booster for a while before you changed the MC, you might have leaked fluid into the booster which could have damaged the booster. Otherwise, I'm not sure why the booster would fail that quickly.

And yes, it does sound like the booster. Do the locks work when you plug the vacuum line?

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-Scott J
1978 240D