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'85 380SL, '99 SL500, '18 C300, '78 911SC, '91 Alfa Spider
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42 Posts
Discussion Starter #1
Hello everyone,

I’ve had a very strange issue pop up after my last drive. Everything was going well until I got a couple of blocks from home and my brake pedal went to the floor, my idle surged, and there was a strange burning smell coming from the motor (almost like electrical burning).

I’ve tried searching for an answer but can’t find anything definitive, if somebody could point me in the right direction that would be brilliant. I suspect it might be something vacuum related, but the strange burning smell makes me think electrical.

Thank you
 

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1985 380 SL
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61 Posts
Sounds like the rears seals in the master cylinder went away, the brake fluid was sucked into the engine via the vacuum booster and burned, hence the odd smell. The resulting vacuum leak causes the high idle. No brake fluid = no brakes.
 

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'85 380SL, '99 SL500, '18 C300, '78 911SC, '91 Alfa Spider
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42 Posts
Discussion Starter #4
This sounds bang on, and what I was gathering through researching the issue. The only strange thing is that I can’t really see any loss of fluid in the reservoir. I’ll replace the master cylinder (they seem relatively inexpensive, pretty straightforward, and necessary at my mileage) and see if that solves the issue. Thank you!
 

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1984 380SL
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2,343 Posts
This sounds bang on, and what I was gathering through researching the issue. The only strange thing is that I can’t really see any loss of fluid in the reservoir. I’ll replace the master cylinder (they seem relatively inexpensive, pretty straightforward, and necessary at my mileage) and see if that solves the issue. Thank you!
MC has two chambers, one front and rear.
Most likely the rear chamber is empty. Remove the master cylinder and look on the inside of the booster. It will probay be full of fluid.

You'll need a new booster and MC sadly.
 

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'85 380SL, '99 SL500, '18 C300, '78 911SC, '91 Alfa Spider
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42 Posts
Discussion Starter #6
MC has two chambers, one front and rear.
Most likely the rear chamber is empty. Remove the master cylinder and look on the inside of the booster. It will probay be full of fluid.

You'll need a new booster and MC sadly.
Oh no, really? I’m not getting a stiff pedal, but does the fluid entering the booster alone necessitate replacement? No chance of just swapping the master cylinder and seeing if that’s the remedy?

Prices of boosters are frightening...

Thank you for your insight
 

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1984 380SL
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The FSM makes an allowance for 100CCs of brake fluid into the booster before the diaphargm is compromised.

Anything more and you're playing Russian roulette.

Suck it out and see how much is in there. Given the height of the tube against the booster itself, id say the outlook is grim.

If it helps, you can get reman'd brake boosters from NAPA auto parts for very affordable prices.
 

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'85 380SL, '99 SL500, '18 C300, '78 911SC, '91 Alfa Spider
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42 Posts
Discussion Starter #8
The FSM makes an allowance for 100CCs of brake fluid into the booster before the diaphargm is compromised.

Anything more and you're playing Russian roulette.

Suck it out and see how much is in there. Given the height of the tube against the booster itself, id say the outlook is grim.

If it helps, you can get reman'd brake boosters from NAPA auto parts for very affordable prices.
That’s great to know, thank you very much!
 
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