WHOA!!! Brake failure!! W124 - PeachParts Mercedes-Benz Forum

jbach36 · #1 12-16-2006, 04:27 PM

......(This post was added on to, with Part II of the story toward the end of page 2)

Major problem. Backed out of the garage on the first start of the day, hit the brakes a few times as usual on the way out and down the nearly-level driveway, out onto the street, and all was well. 1991 300d turbo, with 98k on it.

Put it into forward, down the short (maybe 30 yards) and very gently sloping road to the 4 way stop sign. I hit the brakes, and the pedal was hard. The brakes were braking some, but not stopping the car!! I pushed harder, but I don't think the pedal went down anymore; it was just a hard pedal (not soft and going to the floor). The harder I pushed, made no difference in the stopping ability. I overshot the stop sign by 8 feet. I could have killed someone!

Backed the car up to take it home, it braked just fine. Checked the brake fluid: it was full, and we just changed it some 6 months ago. No leaks anywhere. Tried it a few more times, all was well. Perfectly normal.

I don't experiment with my brakes. I took it to a mechanic's place, he'll work on it on Monday. Anyone know what's going on?

Thanks,

Jeff
1991 300d, 98k

Matt@Revolution · #2 12-16-2006, 05:22 PM

Sounds like you were working without power assist in your braking system. Either failed vacuum pump, large vacuum leak between pump and brake booster, or failed brake booster. Mosst common problem is a broken line from the booster to the vacuum pump on the front of the engine.

meangrean · #3 12-16-2006, 05:49 PM

As sayed by Matt, It is most likely your Vaccum pump.One of the valves inside is stiking. Possible a main leack,but if the brakes worked fine before and nothing was done under the hood to brake the main black hose from pump to booster than i think it is the pump itself.The is a repair kit available to rebuilt the pump,or just replace it

Hatterasguy · #4 12-16-2006, 07:08 PM

On the old 603's that was usualy the first sign the vacuum pump blew apart.

Sounds like a vac leak somewhere.

jbach36 · #5 12-17-2006, 12:11 AM

Could it be the vac pump (or other stuff mentioned) if the brakes have worked fine since then? I checked it afterwards for operation, all was well. Drove it to the repair shop, not a single problem.

Jeff

hihosilver · #6 12-17-2006, 09:37 AM

did you let it warm up as the vac system might take a little time if you didn't drive it for a while and locked and unlocked the doors a few times in between starts

Ara T. · #7 12-18-2006, 12:57 AM

No, you shuoldn't ever have to warm up your vacuum system. You have a vacuum kink in your system it sounds like. Your brakes will work in this situation. You just need to stand on them. The least you can do is inspect all the tees and connectors between vacuum lines, those get brittle after years and years.

A. Rosich · #8 12-18-2006, 09:48 PM

Quote:

Originally Posted by jbach36

Put it into forward, down the short (maybe 30 yards) and very gently sloping road to the 4 way stop sign. I hit the brakes, and the pedal was hard. The brakes were braking some, but not stopping the car!! I pushed harder, but I don't think the pedal went down anymore; it was just a hard pedal (not soft and going to the floor). The harder I pushed, made no difference in the stopping ability. I overshot the stop sign by 8 feet. I could have killed someone!

Just one question to clarify the picture: after you sensed that the brakes were not working properly did you hit the emergency brake???

I ask this because once I was driving a CL 600 (1997 V12) which lost complete brake power as I was approaching a 45° curve at about 60 m.p.h. on a two lane road. The brake pedal went to the floor (found out later the system had a severe leak). I inmediantly slammed the emergency brake and the monster stopped well before the turn started. No panic.

Benz300 · #9 12-18-2006, 11:09 PM

yea sure sux that a mercedes claiming to be one of the pioneers in braking system fails to brake ! totally sux....

Chas H · #10 12-20-2006, 12:22 PM

What part was replaced?

jbach36 · #11 12-20-2006, 01:35 PM

He replaced the booster. It's probably a tough diagnosis anyways, because the brakes failed just that one time. But one time is all you need to go off a cliff. So they worked fine afterwards, and it was probably difficult for him to tell or to see exactly what the problem was. But he surmised it was probably the booster failing, so he replaced it. He thought the vacuum hose was fine. If it fails again, then I'll know it's something else. If I'm not dead, that is.

Hirnbeiss · #12 12-20-2006, 03:20 PM

Mercedes' are designed to crash into things

Benz300 · #13 12-20-2006, 04:00 PM

Quote:

Originally Posted by jbach36

He replaced the booster. It's probably a tough diagnosis anyways, because the brakes failed just that one time. But one time is all you need to go off a cliff. So they worked fine afterwards, and it was probably difficult for him to tell or to see exactly what the problem was. But he surmised it was probably the booster failing, so he replaced it. He thought the vacuum hose was fine. If it fails again, then I'll know it's something else. If I'm not dead, that is.

If the brakes were fine after that one time, what concluded that the booster was the issue ? since the before and after replacement of booster gave same results. meaning brakes were working fine...

Hatterasguy · #14 12-20-2006, 10:39 PM

If it was the booster your brakes were fine, they just needed some grunt.

Driving non assist brakes you have to put your foot in them.

Ara T. · #15 12-20-2006, 10:55 PM

It does freak a lot of people out though, with good reason. The family of someone I used to go to HS with junked their late 70's 300D because it had a vacuum problem (maybe the pump was bad). He freaked out just like the poster did when his brakes lost assist.

My last summer job though I drove an old Ford truck around with no power steering and no power brakes, kind of makes you appreciate the things you take for granted in a car.