I replaced both front and rear brake pads less than two years ago and they should be almost as new, as I don't put more than 2,000 miles per year on this car. However, I'll take a look at both the pads and calipers over the weekend. I doubt it, though.

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1979 240D, W123, 105K miles, stick, white w/ tan interior.

Done that, I built my own..

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1979 240D, W123, 105K miles, stick, white w/ tan interior.

I've checked the line from the IP to the vacuum switch and all other vacuum lines and they are connected tightly.

As far as the possibility of having stuck calipers..I've lifted each corner, then spun each wheel by hand while someone else applied the brakes intermittently. Both front calipers are not stuck..the wheels spin freely and then they are firmly blocked (unable to spin them by hand while brakes are applied). So the front calipers seem to work properly, but braking power is just too weak to stop the car sufficiently well. The rear wheels spin freely by hand and applying the brakes has no effect whatsoever on them (perhaps due to stuck calipers??).

Does this mean that I need to look into the vacuum booster as far as the front brakes go? It could be that the new master cylinder was missing the washer at the seal or something?

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1979 240D, W123, 105K miles, stick, white w/ tan interior.

How old are your rubber brake hoses? I have read similar type problems, when the hoses are old and start breaking down internally....

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No idea how old they are..They are the same which were in the car when I bought it in 2005.
What I'd like to get some feedback on at this point: is it worth it/does it make any sense to take the master cylinder off once again to insure there is a gasket at the seal? Or are the symptoms I have described above unrelated to that? Thanks so much guys..

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1979 240D, W123, 105K miles, stick, white w/ tan interior.

Probably not. If you left out the gasket, you would have a very hard pedal.

Lack of shutoff function indicates you have something going on with your vacuum system that wasn't there before. However, I believe it's only tangential to the brake problem.

Since you don't know how old the brake hoses are, and they aren't expensive, I'd replace those and see what happens. Some people recommend replacing them every six years whether you're having problems or not.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

I took off the master cylinder, yesterday, and found the gasket at the seal was missing, so I installed the one from the old master cylinder. To make sure, I bench bled the master cylinder once again before installing.
With the gasket in place, now the engine shuts off normally via the ignition key.
Braking power seems improved, but still not up to normal. Also, I discovered a leak at the RL caliper nipple, which probably accounts for the rear wheel brakes not working. It seems the thread of that nipple was damaged and it no longer screws in tight. Now, with the gasket in place and the vacuum booster therefore working, pressing the brake several times has the level of the rear brakes reservoir go down.
Any ideas on how to fix the nipple screw short term?

@Skippy
Before installing the old gasket, the brake pedal did not feel very hard at all, even though the vacuum system was obviously not working. Now, with the vacuum pressure again on, the brake pedal is light and it travels all the way to the floor, pumping it twice achieves some reasonable braking, but still not up to par. Can you point out specifically which brake hoses you are referring to? Thanks..

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1979 240D, W123, 105K miles, stick, white w/ tan interior.

Not what I would have expected, since I've driven one with no vacuum assist and the pedal was very hard. I'm talking about the hoses that run from the ends of the hard lines to the calipers.

As for your stripped bleeder screw, the quick and dirty fix would be one from a junkyard. Since it fits in your pocket, it just might be a free part if you know what I mean.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

OK, I got it. But if those hoses were damaged, wouldn't they be leaking somewhat with the level in the reservoir going down after a while? Nothing like that as far as the front lines go.. Also, the brakes were functioning perfectly well until there was a failure in the master cylinder, which probably I brought about by pressing the brake pedal all the way down when trying to bleed the system that way (by bleeding air at the nipples with an assistant who was pushing on the brake pedal). The master cylinder being very old, it likely got damaged that way.

It's a bit of a mystery to me why it still behaves this way after replacing the master cylinder and ensuring there actually is a gasket at the vacuum booster seal. I think it unlikely that the new master cylinder be also defective, which could explain what is taking place here, but I doubt it.

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1979 240D, W123, 105K miles, stick, white w/ tan interior.

The hoses break down internally. They don't leak, but they don't flow fluid properly either. Usually this results in a stuck caliper. However, you're having a rather unusual set of symptoms.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

At one point last week, I placed the car on stands to take a better look and, lo and behold!, I instantly knew something was very wrong! I finally noticed that the rear calipers were mounted upside down (with the nipples on the bottom instead of on top). No idea who did that, probably the previous owner or, likely, an absent-minded mechanic.
Switching calipers to the opposite sides has resolved the problem. The system could then be properly bled and the brakes have worked perfectly.

The only reason I was able to drive the car like that for almost nine years, I think, is that ,when replacing the brake fluid, I never let the level in the reservoirs go all the way down (except for that one recent time), so basically air never got into the system. But once it did it caused the problem I experienced, since it simply could not be properly bled with the nipples upside down.

So, in the end, it turns out the old master cylinder was working perfectly fine, there was no need to replace it. I spent so much time over this thing..built a power bleeder I had really no need for, got a new MC which wasn't necessary. And all that aggravation. Always something to learn (the hard way).. :-(

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1979 240D, W123, 105K miles, stick, white w/ tan interior.

Guys, an earlier post says you just press fit the reservoir back on the new master cylinder. But how do you remove it in the first place? Do you just muscle it out, screw it out - what?

PS - this is for my 81 300SD but I assume the master cylinder is the same.

You have to just pull it straight up....there is a chance it might tear...I have pulled a few off at the JY's but I tore one on a w115...

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Great; thanks for the response.