I'm trying to diagnose a recent change my brakes went through in the "feel" at the brake-pedal. All of a sudden the pedal one day seemed to sink down much further before engaging the brakes, to the point where it felt like the pedal barely worked. There was also a "judder" kind of sound as if rubber were slipping inside the master cylinder... as if I guess the piston-seel had been compromised?

So.. today I finally had some time to try looking for some culprit for this new condition of the brakes. When I checked the reservoir I noticed that there was almost no fluid in it, but that there seemed to be fluid in another part of the reservoir.. almost as if there are two reservoir chambers? I re-filled the main chamber and applied a pressure bleeder kit to the top of the fill-hole to see if I could find if any of my hoses or calipers or brake lines were leaking and found nothing in that departement. Up top on the reservoir though I found something strange... one of the "rubber stoppers" that fit over the top of those two brake-light switches (I think that's what they are anyway), was split and would hiss when air-pressure was applied to the reservoir... Under the rubber topper thing, there seems to be a valve of some sort there. Is it critical that these rubber stoppers be able to hold pressure? Why would it hiss and sputter a small amount of fluid? Why do these little valves exist?

After re-filling the reservoir the pedal response seemed to come back a good amount, but not to what my memory seemed to be of what it used to be. I drove around and was able to stop better, but there was still a little of that juddering sound.

Does anybody have any ideas as to what this whole problem could be about? I wish I had checked the brake reservoir before this had all happened, but I never had since I had bought it... so I don't know if this is a sudden loss of a lot of fluid in the reservoir, or the brakes were working great even though there hadn't been a good level in the reservoir?

Oh... and what are the chances something has gone wrong with the the vacuum powered assist? Like say the vacuum power has been lost somehow?

Thanks everybody for your input and help.

There is no "almost" involved. There is a separate chamber front and rear brake systems. That way, a leak in one place will not cause a total loss of fluid, which would be followed concurrently by a total loss of hydraulic brake capability.

A vacuum problem on a 30+ year-old W123 would not be unheard of.

If the Vacuum goes bad or the Power Brake Booster itself goes bad it causes the Braking to take more physical effort; "hard pedal"

If the Rear Chamber; the one closest to the Brake Booster was the one that was empty it could mean thet the rear Master Cylinder Seal is leaking and the Fluid is leaking into the Power Brake Booster. (You can loosen the Master Cylinder retaining nuts and scoot it forward a little (leaving all the tubings connected) and see if it is wet behind it and inside the Power Brake Booster.)

Also when you run out of Fluid as you did you should re-bleed your Master Cylinder. If that does not work you should bleed the Brake System.

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84 300D, 82 Volvo 244Gl Diesel

with your powerbleeder, I'd flush the MC, and then flush each caliper starting at the rear.
odds are good you have a leak somewhere, like a flex line, or a leaking rusty steel line. also likely is the front pads are VERY worn, and need replacement, resulting in low fluid level due to excessive piston travel on each wheel.
further diagnosis is needed to figure where the fluid went.
get out the jack stands and floor jack!
those rubber "stoppers" are part of the warning system that's supposed to let you know when you are out of fluid. the're hissing because they are cracked and worn out (30+ years will do that to rubber...) you can replace them, or replace the entire reservoir. (parts cars are a godsend!)