Yellow light and squishy brakes
 

Drove the '85 300 td to town last night and the brake pedal went soft and about 1/2 way down a couple of times, lasting a minute or two, then became firm again. Not at that exact time, but intermittently throught the drive the yellow light on the dash next to the brake warning light came on. I has a round circle with dotted lines around the perimeter. On the way home ,12 miles it never came back on. Are the two related? I don't see a traditional vacuum pump anywhere... Where is it? Also, last but not least, what repair manual do I want to buy? All I have is the owners manual in German. Thanks

No need to buy a repair manual. It is available online for W123 chassis:

http://www.startekinfo.com/StarTek/outside/12265/?requestedDocId=12265

The car has a vacuum pump, it's up at the front of the engine on the right hand side near the power steering and A/C compressor. Follow the thick vacuum line that runs from the brake booster to the front of the engine and you'll find it.

However, it's not the cause of your problem here. Vacuum problems would lead to a hard brake pedal, as the booster uses vacuum to assist braking.

A soft pedal and the light being on means that the fluid is likely low. Which is usually indicative of a leak.

This light is the pad indicator light.
http://www.peachparts.com/diy/w123br...glights/01.jpg

There are sensors in the pad that look like this
http://www.mbca.org/sites/default/fi...ake%20pads.JPG

As to why the pedal is going soft. sounds like maybe a fluid leak. the red brake light will come on when the fluid in the reservoir gets low.

du nein kennst der deutsches Spraech, aber bist ein Mercedes-fahrer? Ach was! :D

I suspect master cylinder worn... typical failure -> soft pedal and gets normal if you pump...

Thanks for all the help. Is a kit available for the master cyl or do I have to replace it? Drove it a bit after work today and no issues with the light or soft pedal. I am old enough to know this wont just go away. First thing I fix on an old stray vehicle that follows me home is the brakes. The rest is just frosting.....

replace the master cylinder ASAP, the rubber seal is more than likely broken

Nothing in this post suggests the Master cylinder is bad or needing anything more than fluid...

the FIRST thing I'd do is get a quart of DOT4 fluid, then a set of front pads, (textar would be my choice) and a fresh pair of wear indicator wires. then I'd pull the front wheels, and replace the pads, and inspect the rotors. if the rotors are worn beyond the wear limit, I'd change them as well. next, empty the reservoir and remove it, then clean it out well, and reinstall it. next I'd fill it with the new fluid, and hook up my motive power bleeder, and pressurize the system, and bleed out all air, and flush all old fluid out of the system.
then I'd inspect the rear pads and rotors and see if they are worn as well, and replace as needed.
done.
solid MB breaking again.

Cleaned up the front brakes and there were some pretty bad rust 'footprints' on the rotors where it had sat for 5 or so years. I sanded them out and worked the pistons back and forth a bit. Pads were about 3/16th thick by eye. Rotors measured 11.8 mm. Now off for a test drive, maybe show off our 1000.00 treasure to my buddys. Swapping out the brake fluid tomorrow. Just welded a sleeve onto the muffler where it broke off on the way home from buying it. A donut had fallen off and it had fatigued. Now it's double strength in that area, plus a new donut. Well actually a long piece of electrical wire zigzagged back and forth until I get some new donuts this afternoon. Figure I better bring a tool kit......:D

Hay bailing poly twine wrapped around & around works well. Problem is once its done it gets forgotten until it breaks ~ normally about 20,000 miles.

just talking from experience; i bought an SD that sat for 7 years, brakes went out during 1st drive after bringing it back alive (flushed all lines). took out MC to inspect; back rubber seal was in pieces from sitting in the old fluid so long

If the problem doesn't go away after your brake fluid change, including proper bleeding, it's probably time for a master cylinder. The internal seals go, so there's no leakage, but it won't develop pressure the way it should. I did mine in about 45 minutes today, then ran out of daylight so I'll be bleeding it tomorrow. I got the part from this site. They had three or four manufacturers available and I decided to try the $38 Meteli cheapie.

Wow! That's a good price. I was gonna drive my new prize to Seattle tomorrow, but I feel it's just too new to us and I'd be pushing things to race down the interstate. I guess Ill take the '85 landcruiser even though it gets half the mileage. On our test drive today we stopped at Frankie Gate's place to show off the 300 td. He's a dump truck driver and heavy equipment operator here on the island who loves the diesel Mercedes. I was told he has a lot of parts, and he's a guy I have known for years. When we left his place, we had 4 hub caps and an amazing grille, that might not be 'year correct'... but I don't care. He wanted 40 bucks for the 4 hubcaps and 40 bucks for the grille. I made him take 100.00......and that included a rear bumper with all the rubber next tim I stop by..... The bling is nice, but I gotta have brakes first.