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#1
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Erratic brakes, 1981 300SD
Just got this car last month and am doing my best to learn it's quirks. Today, once I got it running again I noticed a development in my soft brake issue.
Actually, to go back to the begining, the brakes were always too soft since I've had the car, one of the reasons it failed inspection. One day while rolling the car into a different spot on the street in neutral when it wouldn't start I noticed the brakes were hard for a minute. My previous diagnosis had been maybe a bad master cylinder, but this made me question the booster. When I got around to looking at it, there was no brake fluid in the front reservoir. So today, as I am test driving the newly started vehicle, I noticed the brakes were their usual softness, except for one point when they suddenly became rock solid, and luckily I was in the parking lot at autozone so I stopped. Didn't know what to make of it and it didn't do it on the ride home, prolly a not so safe move. When I got home, I slowly added some DOT 4 brake fluid to the reservoir until it was full, pumped the brakes for ha-has and they felt good, though I knew it wouldn't last. Once I started the car, the softness returned. To my untrained eye, this seems like a bad master cylinder that's starting to leak into the brake booster possibly via the o-ring that conneects the pair, but I was wondering if a leaky vaccuum pump gasket would have anything to do with this. I don't have any other vaccuum related issues that I'm aware of, except maybe some hard shifting but that's only when it's cold. I'll check for leaks on the booster to see if the o-ring is leaking, and I'm also bleeding the brakes tomorrow, so maybe that'll do the trick. Curious to hear someone's input. ~Brian |
#2
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I probably would change the master if you are sure you are bleeding them correctly. Actually if it were a bleed only problem it would only get solid if you pumped them up.
__________________
[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. |
#3
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At the risk of sounding old and mechanically conservative .... which I do happen to be...
You need to stop putting yourself and others at risk until you resolve the brake issue. On that old a car ... and given the relative importance of brakes... I suggest you put a new master cylinder and a new brake booster on your car. They will last a long time and serve you well while you fix the other quirks in your car... You need to check for air in your brake lines. In addition to those other items which can cause changing symptoms air in the lines can also ....and if they are all squeegy they can really mess up your diagnostic flow. Seriously, it is an old car and multiple things can be wrong at the same time... |
#4
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I hear ya leathermang about the safety issue. That's why I haven't driven it since that test drive. I've got plenty of other work I can do on the car until more new parts arrive. If bleeding the lines fixes the symptom, would you consider it safe to drive?
By a "bleed only problem" t walgamuth, do you mean that if the lines just needed to be bled they would still be able to be pumped up and stay firm? I can pump the brakes up but the pedal sinks if that clarifies things. I know there's a lot on the forum about brakes but I hadn't quite found any posts with these multiple symptoms. I'm going to bleed the lines today and take a closer look at everything see what I can find. Thanks for your replies. ~Brian |
#5
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Check all the brake lines as well. The old hoses can burst and the metal lines can rust from the inside and leak.
__________________
"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy" Current Monika '74 450 SL BrownHilda '79 280SL FoxyCleopatra '99 Chevy Suburban Scarlett 2014 Jeep Cherokee Krystal 2004 Volvo S60 Gone '74 Jeep CJ5 '97 Jeep ZJ Laredo Rudolf ‘86 300SDL Bruno '81 300SD Fritzi '84 BMW '92 Subaru '96 Impala SS '71 Buick GS conv '67 GTO conv '63 Corvair conv '57 Nomad |
#6
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I took quick look at the hoses, they all seem dry. The front ones a little wet, but I've got other leaks up there that are more likely the source. I took a look at the brake booster and it's got rust in the pattern of running down the front where the MC attaches, so I'm going to order a new o-ring and bleed the lines for now, see where that gets me. I'll check for fluid in the booster when I replace the ring obviously. The brake lines are on my list of PM when I get to that stage. Right now, I could use a running car that drives and stops. But first I've gotta go ship out an eBay sale so I can continue to justify all of this money hemorrhaging out of my savings while I'm on vacation...
~Brian |
#7
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Your booster has nothing liquid to run down the front of itself... it does not drool.
So your description means to me that your master cylinder is leaking...and that o-ring you ordered really has little or nothing to do with that... nice to replace.. but not structurally at issue right now. Tom already said it... put a new master cylinder on it. It is possible that your rubber brake lines have gotten soft.. not common.. but they are probably pretty old and you do not know what other types of things have been sprayed on them over the years... so if you are sure you have clean fluid, have the air out, new master cylinder and power booster with no vacuum leaks in the system going to it..... and the pedal still seems to mush...then certainly look at replacing the rubber lines. Those ' other ' leaks ... rubber brake lines are meant to withstand water and stuff like that.. but not long term soaking by other harsher fluids... |
#8
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droooool
I know the booster has no liquid to drool, I meant the master cylinder might've been drooling on the front of the booster, which still means I need to replace the MC. I didn't order that ring. Still gotta get to bleeding the lines sometime today. I'll update my progress when there is some. About the brake lines, with that info in mind I should probably prioritize fixing the leaks before replacing the lines. Thanks!
~Brian |
#9
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Quote:
If you are going to keep the car, want to sleep better at night, and save expensive fluid.... just go ahead and change out the rubber lines before you start bleeding for the other changes... Good luck, |
#10
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Quote:
Exactly. While I did not mean that weak hoses was the cause of the soft pedal, If you fix the master cylinder the nexe weakest part will fail quickly. better change the rubber bits and make sure the hard lines are OK before you think you are done. I had a similar issue on the old CJ5 the master cylinder would slowly leak fluid and the pedal would go from hard to soft to oops, no brakes! In my case it did turn out to be a hard line tothe rear brakes.
__________________
"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy" Current Monika '74 450 SL BrownHilda '79 280SL FoxyCleopatra '99 Chevy Suburban Scarlett 2014 Jeep Cherokee Krystal 2004 Volvo S60 Gone '74 Jeep CJ5 '97 Jeep ZJ Laredo Rudolf ‘86 300SDL Bruno '81 300SD Fritzi '84 BMW '92 Subaru '96 Impala SS '71 Buick GS conv '67 GTO conv '63 Corvair conv '57 Nomad |
#11
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I know I am faaaar from done with this car. It's only just begun. This car is my crash course (no pun intended) in MB car repair. I've got a lot on my plate so far with this car, but luckily it still seems managable. Brake work is probably the scariest thing for a noob like myself to tackle.
Hopefully I can sort this out tomorrow and place my last order for the week. Made a couple of sales on eBay so I can justify one more big order to get it back on the road. Would you guys trust a good used MC? ~Brian |
#12
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On diesel mercedes yahoo groups,there was a post on bleeding brakes overnight,by letting a stick hold brakes in.
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1999 w140, quit voting to old, and to old to fight, a god damned veteran |
#13
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If you were the only one driving it...and I was having an affair with your wife.. and you had lots of paid up life insurance..... sure....
otherwise... . NO. |
#14
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Quote:
The question which comes to my mind without even hearing the specifics.... is : What is wrong with the old regular way (s) of bleeding the brakes ? |
#15
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Theres a good Benz Mechanic that uses this procedure for hard to bleed brakes.I've never had that problem.
__________________
1999 w140, quit voting to old, and to old to fight, a god damned veteran |
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