Brake failure W124 Part II
 

......(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

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.

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

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

Sounds like a vac leak somewhere.

Vac pump?
 

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

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

Warm up
 

No, no, no, no, NO. I'm not going to be IN a car that needs to be warmed up before stopping well. I mean, I went through a stop sign by 8 feet. And that was after I backed out of my garage and driveway and had already hit the brakes a few times.

No brakes is a serious issue. I could have killed someone, or run off a parking deck, or run a red light and gotten hit broadside. No, I'm not going to even OWN a car that needs the brakes to be warmed up before driving. Sorry.

So...what are you going to do ? Sell it and let some else have the problem or are you going to fix it properly and keep it...third option, crush it.



.

<<No, I'm not going to even OWN a car that needs the brakes to be warmed up before driving. Sorry.
>>

The Lone Ranger was not implying such..he was simply asking a diagnostic inquiry.

Ah, had this problem, in our case it was start wagon, backup out of garage and no brakes, then if you turned it off it would not shutoff because no vacuum. Wait a 30 seconds and everything was fine. After determining it was not a problem with the booster, or the vacuum line we replaced the vacuum pump and everything worked as usual. We did note the pump was slow to build vacuum when it was cold, which led to the behavior, we had thought it was the check valve in the booster line but this was not the case, it seems the valve is there to provide enough vacuum for apply the brakes to shift out of park after starting the car, but once that happens the pump should have pulled enough vacuum to ensure the brakes work.

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.

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.

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

Sell it to someone else??? And emergency brake..
 

No, I'm not going to sell it, so someone else has the problem!! Would be my luck the person he would kill from not stopping would probably be me!! I had it fixed today at the mechanic's. $170 part that I bought and provided to him, $125 for him to diagnose and fix the problem. Hopefully it's fixed once and for all.

As to the other question someone asked, did I hit the emergency brake? The answer is no. I was basically coasting down the slight slope toward the stop sign, just 3 houses away from me (thank God). So I was only going 20 mph towards the 4 way stop sign. I might have hit the brakes 20 feet prior to the sign. It happened all so fast, hitting the brakes, then wondering if I hit the pedal, as I wasn't stopping. I looked down, foot was on the pedal, I pushed harder, but the pedal didn't seem to go down anymore; just a hard pedal. By the time I thought of hitting the emergency brake, I was already past the stop sign.

Thanks,

jeff

What part was replaced?