Been lurking for a couple weeks. Great stuff on here. But now I've got a serious concern (at least to me!). Today I started up my car, after stopping at a store for about 15 minutes and drove down the block. Edit(Meaning the car systems were all warm) When I came to the next (very busy!) intersection I put on the brakes and the pedal sunk to the floor. Finally the front brakes locked up and I skidded for about 50-75 feet and launched the car over the curb, which was a better option than the traffic in the intersection. Didn't think about slamming on the parking brake at the time, but fortunately, other than possible damage to the tire sidewalls where I jumped the curb and some new gray hair, we're unscathed.

The master cylinder is still full; I didn't see any evidence of brake fluid leaking from any lines; and after my heart stopped pounding, I slowly drove the car home on back streets and the brakes worked fine (once I "pumped them" after restarting the car). I have never had any brake problems in the past. The brakes were redone about 2 years and 6,000 miles ago (car wasn't driven for about a year until a month ago).

Any ideas of what I should look for next? I thought my next step might be to change the fluid and bleed, but to have total rear brake failure like that and then to seem fine afterwards makes me wonder what else it could be.

The car is a 79 300D with 102k on the clock. The car has been in my family for probably 20 years, with little use; hence the low mileage.

failed internally master cyl.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Although Tom might very well be correct, I'd take a look at the reservoir carefully and make sure that both chambers are full. There is a rear chamber, for the rear brakes, that is difficult to see and it's easy to think it's full when it is not.

Make absolutely sure that the rear chamber is full.

Then, if it is, before changing the m/c, you might want to bleed the rear brakes just to see what you get. Probably won't amount to anything, but, if there is air in the rear system, the symptom is identical to what you described.

If both of these items fail to produce results, then it's m/c time.

If the pedal sunk to the floor, I don't think it was a rear brake failure, even if the front's ended up locking up. If the rear brakes alone failed, the pedal should still be pretty firm (I believe) and you'd stop pretty well with the front brakes alone since they do more of the braking.
I also think it is an internally failed master cylinder. I'd replace it or rebuild it and see what happens.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Thanks, I'll look at the rear resevoir tonight and try to bleed. Any recommendations about a m/c rebuild kit vendor and manufacturer? I haven't done one in years, but I recently did an in-tank fuel pump replacement on my truck and ended up buying parts kits from 3 different stores because none of the kits had every piece needed to complete the job. Nothing like getting everything apart and needing to get one more little part to finish it.

There have been a lot of discussions regarding rebuild kits of late. AFAIK, you can't find a rebuild kit for the m/c due to liability issues. Most of the vendors simply sell a rebuilt m/c and you return the core.

Perhaps the liability issue is why i have not noticed rebuild kits on ebay lately as someone suggested. They usually go for about 10.00 plus shipping when on the site. I have seen complete aftermarket master cylinders for 29.00 plus shipping in the last month or so on ebay as well. I should have brought a couple of spare repair kits myself. The first kits I purchased were great. Think if you check vendor sites if low fluid does not turn out to be the problem someone should have one. The kit fits a lot of various mercedes models so look under the 450sl type of cars as well.