Does anyone know if the brake booster and master cylinder from a late model 85 or so 300D AT will fit on an 81 240D manual? I have found what looks to be a fairly good on in a junkyard and it would probably be way cheaper than the 500 or so bucks it would take to source all the components online. I know there could be a fuse box clearance issue with the larger booster but I'm more concerned about actually mounting it to the firewall. And on that note, how do you actually mount the whole contraption? Is it just a couple of bolts thu the firewall as it appears or is there more to it?

- Peter.

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2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

First I agree. The replacement rebuilt or new booster is too high in price. A good used one is the way I would go as well. . To check fit of the 85 booster just do a comparison part # hunt for both years. I suspect they are interchangeable. The masters should interchange with little effort as well I suspect. You can transplant your plastic brake fluid tank onto the 85 master cylinder if it is different. Even the part numbers for the two master cylinders should be verifiable.. Takes the guesswork out.
I am not a big fan of used master cylinders though if they have sat around for a long time unused. That is just me and others will feel differently. They used to have a kit that I found very useful for those masters. As low as around ten dollars for the last ones I brought. If you can locate one cheap I would buy and install it. It is a very easy job and you then know you have new rubbers in there. For liability reasons I have been told the kits are harder to find now but still out there somewhere.
Okay I just had a look at the adsit site. A seller of mercedes parts. They are not a stellar place to buy things but I just used their online cataloge as a refference. The boosters from 79-85 are the same. The master cylinders indicate a change of part number probably in the mid season of production in 1981. The later part Number appears to be 043-2300 and they are the same as the 85 master. The earlier 1981 master is 043-1600. So you have a little checking to do. The master cylinder if the earlier part number may or may not work depending on why they changed the number. Others will know more than myself needless to say. The primary reason I responded is to illustrate how easy it is to cross check part applications from different years as it only took me a minute or two.
You have four nuts to remove from under the dash to get that booster out. Plus the pedal connection needless to say. One of those four nuts in my opinion takes a bit of effort to do. I remove the fuse panel from now on to get the hard one instead. It allows you very easy access to another as well through the fuse panel opening. I do not recommend this approach as have not done it other than once to remove one off a wreck. Make sure you use that approach to get the used one from the other car though as you can do no damage. It is already junk. It makes life much easier. Then you can decide if you want to do that as well to install it on your car.
Please disconnect one terminal from the battery if you do decide to do that on installation and work very carefully. Good general policy is to make it a habit to remove a battery cable for almost all jobs around wiring and plugs anyways. Required to keep you safe on quite a few other types of jobs as well. Hope I have given you a little food for thought.

The master cylinders can be interchanged- I took one from a 240 and put it into my 300.

Not sure about the booster.

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Andrew

1989 Volvo 745- 202K

I've heard the cost difference between a new unit and a rebuild kit (master cylinder) is not enough to do the work of rebuilding. Check *************** for part #'s, prices. Be sure to obtain all gaskets. There are 4 bolts holding booster to firewall, as noted, one can be difficult to get to, but can be done.

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83 SD

84 CD

I don't think I would reuse the master cylinder. My car came with a spare used master cylinder. It had been removed in working condition and stored indoors for a couple years. It was stuck pretty solid and filled with brownish brake fluid. I took it apart and found the cylinder is rusty and there are a couple of nice ridges where the rubber seals had rested all that time.

I can only think that something sitting outside for a while has to be suspect - especially something as vital as a master cylinder.

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1984 300TD

We put a new booster off a 84 300TD and put it on a 80 240 d .

did not take the master off either. I did not want to do a brakle bleed session. If its just the booster and your master is not leaking .. I'd go for just the booster. .. and yes it will fit.
Just bent it down(master cylinder) and removed the old booster and replaced with the good 300td booster.
Hardest part was the 4 bolts on the interior firewall... gear wrenches made that easier as well.

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81' 240d 4 spd 140k on the clock

If you don't have a set, head over to Harbor Freight and grab one before you try the job. There's a 3/8" extension in the set that is around a foot long. It'll make short work of that nut that is hard to get to, especially if you have an air ratchet.

Jay.

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er