So in the last two weeks I've started hearing a steady vacuum hiss near my feet whenever the engine is running, which has grown steadily louder up until today, the pedal has become increasingly stiffer until today, and starting last night the car refuses to shut off with the key (although it has a few times if I stood on the brakes with both feet after taking the key out.)

So I'm guessing this means I probably need a new booster? Or is there some kind of seal or something that could have gone bad? I looked on some of the other brake booster related threads but couldn't find anything definitive. I would like to fix this as soon as possible as brakes are kind of important to me

Thanks folks

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint

Is the Engine shutting off OK?

The Shutoff Valve on the Steering Colum and the Vacuum Lines to your Door Lock to the left and on the Floor are the next 2 closest Vacuum Sources to You when you sit in the Drivers Seat.

There is an O-ring between the Master Cylinder and the Brake Booster but I am not sure how well you could hear the sound from that if it was leaking.

There is a test in the Service Manual where you make a Fitting to attach right at the Brake Booster and apply Vacuum to the Booster and see if it leaks. But, I don't know the details on that.

It is surprising that Few Members have had Brake Booster issues considering the age of the Cars.

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84 300D, 82 Volvo 244Gl Diesel

I am thinking if you carefully disconnect the Small Vacuum Hoses/Lines from the main Vacuum Line and put caps on the Nipples that would cut off the Vacuum to everthing but the Brake Booster.
If you still hear a Vacuum Leak it has to be the Brake Booster itself or the Master Cylinder to Brake Booster Seal.

If it is the Mastercylinder Seal the leak sound would be louder if you open the Hood.

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84 300D, 82 Volvo 244Gl Diesel

I'll try to check those things out in the next day or two. And the engine isn't really shutting off unless I use the stop lever under the hood

Also, when I open the hood I don't really hear any leaks when I have the hood open either which leads me to think its a leak inside the booster or something along those lines

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint

How is your vac pump? Any oil in the vac tube going to the booster?
My guess is your vac pump has a failed diaphragm & oil has got at the booster diaphragm & its failed.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

Vacuum pump is fine as far as I know

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint

If you disconnect the Main Vacuum Line right at the Brake Booster and cap of the line; or you can Start the Engine and block the line off with your Thumb.
After the Engine has idled a bit have some one turn the Key and to shut it off and see if the Engine shuts down normally.

If the Engine shuts down normally you have a other indication the leak is in the Brake Booster.

You ask if it was possible there was a Seal inside the Booster that was leaking and that is possible.

I have not had any make of Brake Booster apart for 35 years and when I did that in the Valve Mechanism there was a Washer that had flat Metal Fingers in it and Rubber was cast over it and the Rubber had cracked on it. Not a part available from the local Auto Part Store.

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84 300D, 82 Volvo 244Gl Diesel

I think Diesel911 is on the right track with checking the lines to the door locks and shutoff valve first. If you don't have a manual for the layout, just trace the lines back...they're right at the top of the engine and pretty clear to see. Pull the line to the door locks at the rubber connector, cap off the connection, and try shutting off the engine, then the same for the others in turn. If one of those solves the problem, the good news is that it's a pretty cheap fix. Bad news is that it's a royal pain threading the new vac lines through the door. Duct tape, determination and a healthy dose of swearing really helped with that part.

If the seal between the master cylinder and the brake booster is bad, usually you'll see signs of brake fluid leakage...rusty staining leading down from the connection. I think if that were it you'd be feeling brake mush, not hard stopping.

And then there are those rubber connectors themselves to the vac lines. We've had three w123s and had to replace all of those under the hood. They get get hot and develop tiny cracks, and any little hole in the system can cause massive headaches. They're very cheap, too. While you're under there looking at the system anyway, might be worth it just to replace them.

And if all that fails...sounds like the booster. I don't know your feelings on used or reman. parts, but the reason you can't find too much on w123 booster failure is because they are a very sturdy and tend to last a long time on average. It might be worth looking into a used one for the cost difference. Seems like the odds of that dice roll would be in your favor.

Might just remove the fibre panel above the brake pedal. Take a piece of tubing and insert it in the brake rod area. Since you are not hearing it out front in the engine compartment the noise is probably coming out the back of the booster. The tube to your ear wil verify the source if there is any doubt.

Personally I beieve the booster is gone from your description. A tear in the diaphram. There is four bolts securing it and one seemed a little difficult to get from under the dash.

Best still to get whatever tools you need to do it that way but there is another way to get that hard fitting. The care required to do it is why I do not suggest moving the fuse panel but I have done it. Allows access from the front of the car. It is realy not hard but more of an awkward thing to get that one fitting the normal way.

Are the bolts securing the vacuum booster the same as those the brake pedal assembly bolts to? If so then I know which ones they are. I have a parts car I can pull a booster off of so I should be covered there, I guess I just need to find some time to do it. Maybe after I get my w115 back from the body shop in a couple of days I'll try working on it

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint

I only had mine off one time so I don't remember the details except that there is one Nut that a Swivel Socket (13mm???) is easier to use on and there is some sort of Bracket that has a hole in it. You can insert the Socket and Extension through that Hole in the Bracket to get at the Nut easier.

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84 300D, 82 Volvo 244Gl Diesel

Hmm I'll have to check it out I guess

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint

Yes the booster bolts to the brake pedal assembly. There are 4 nuts to remove from the pedals.

I would check the booster with a mighty vac to see if it holds vacuum. Sometimes you would need press down on the brake pedal at various points to stretch the diaphragm to see if you loose vacuum.

If not, then the hissing sound points to the climate control vacuum pods or lines to the pods are leaking or ignition switch vacuum leaking.

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1983 123.133 California
- GreaseCar Veg System

I'm gonna say it is probably the booster leaking because the pitch of the sucking noise changes when I press the pedal, and when I take the key out and the car doesn't turn off, standing on the pedal with both feet until the hissing gets quieter usually lets the engine shut off

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint

Heyyyyy guess what!? Disconnected the vacuum line going to the booster, the hissing noise inside the car went away, and when I plugged the line the car shut off with the key!

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint