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1983 Mercedes 280SL
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242 Posts
Discussion Starter #1
Just a super quick bit of advice: the hiss is especially loud from the driver's footwell. Is this absolutely the servo leaking? Could in no way be the hose or the check valve?

Looking at the diagram, the gasket (highlighted). If that perished would that cause a leak? Surely not.

Any help very much appreciated.
Cheers
 

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1973 450SL
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651 Posts
No, that gasket would not cause a leak. All of the vacuum should be held on the OTHER side of the diaphragm inside of #5. If you hear a hiss when applying the brakes, you may have a diaphragm leak.
 

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1983 Mercedes 280SL
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242 Posts
Discussion Starter #3
No, that gasket would not cause a leak. All of the vacuum should be held on the OTHER side of the diaphragm inside of #5. If you hear a hiss when applying the brakes, you may have a diaphragm leak.
Thanks for your reply. That's what I thought. This hissing in the footwell is when I apply the brakes only leading me to believe it's not the hose. It's not the master cylinder as that's brand new. Do you think I should go ahead begin sourcing a new servo?
 

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1983 Mercedes 280SL
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242 Posts
Discussion Starter #8
Managed to source a good salvage booster. Before I bite the big bullet. It does definitely sound as though the booster is at fault?

When the engine is off the pedal firms up nice. When the the engine is running the pedal sinks to the floor with a noticeable hissing in the footwell.

Is the hiss the smoking gun? Doesn't just need more thorough bleeding?

Any help much appreciated.
 

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It Is What It Is, Dude
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22,841 Posts
Thanks for your reply. That's what I thought. This hissing in the footwell is when I apply the brakes only leading me to believe it's not the hose. It's not the master cylinder as that's brand new. Do you think I should go ahead begin sourcing a new servo?
When the engine is off the pedal firms up nice. When the the engine is running the pedal sinks to the floor with a noticeable hissing in the footwell.


That part is NOT related to the booster.

The only way the pedal can go down that far is loss of hydraulic pressure.

Maybe the new brake master is bypassing, or the brake bleed job not sufficient.

The hissing you're hearing just may be air rushing through the canister's vent port to fill the vacuum behind the diaphragm when the pedal depresses so far past its usual position.
 

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1983 Mercedes 280SL
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242 Posts
Discussion Starter #10
Thanks for your reply. I appreciate it.
... the brake bleed job [might not be] sufficient.
I thought I bled it as well as it could be bled. Perhaps not. If I attempt to bleed it more but proves futile and I need a new booster anyhow, is that fluid then wasted?

Maybe the new brake master is bypassing
What does this mean?
 

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1983 Mercedes 280SL
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242 Posts
Discussion Starter #14
It's when the seals on the master cylinder piston fail to seat properly, allowing pressure to bleed off, and the pedal to fall.
Oh okay. Thanks. For that to be the case would it mean that the brand new master cylinder I've just installed is faulty?
 

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1975 450SL
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2,440 Posts
You may want to go through the bleeding procedure again. Make sure that you get fluid into both chambers of the reservoir, the rear one can be a bit tricky. If possible, use a pressure bleeder, as they are reputedly very effective when ABS brakes are involved. Brake fluid isn't that expensive, and if you get the system properly 'burped' you'll know if the problem was the bleeding or the booster.

Although I did have to replace the booster on my pickup this summer when it exhibited like symptoms. It was easier on an '80 GMC than it would be on a 107, so I would try to eliminate the other possibilities before going in to the dash to get the booster loose. Be careful.

Scott
 

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1983 Mercedes 280SL
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Discussion Starter #17
Hi Scott. Thanks for your reply. I've went ahead and bought another servo. I took the old one off and it's really corroded and unpleasant looking around the area that protrudes though the bulkhead. Managed to find a cheap one. Did use a pressure bleeder (they are awesome). Will report back with any useful info once I've installed the new one.
Cheers
Ben
 
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