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#1
03-21-2006, 03:59 PM
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Just bought my first Diesel,Strange brake problem
Just joined the diesel ranks. Proud owner of a 1978 300CD non turbo.163 thousand miles, near pefect body and interior. one very small tear on drivers seat back. Love these MB coupes.
 Its a white coupe with tabacco color int.?Power Window's,manual seats and polished rims. May also get a full set of painted hub caps which i will sell. Strange problem, he said he just replaced rear pads and tried to bleed rears.Still has a low pedal. He thinks it still has air. Odd that it won't pump up.On test drive pedal goes to floor and slowly stops car.But if i floor the gas for a second pedal is on top and firm.  Can this be a bad vacuum pump?Kind of odd to have 5 percent brakes the acceleate and have 100 percent.  when it gets home i will check vacuum system and maybe bleed brakes anyway. any simular problems from someone here? Thanks Rich
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88 300E White, sold 9-04  88 190E Black 2.3 sold 90 300CE White, Grey sold 12-24-04  92 400E White,Grey Sold 5-06  90 300CE 91K Signal Red,Black Sold 2007 1989 300CE Smoke silver,Tan Sold 1-08 1993 300E 3.2 M104 motor 1991 300CE White,Grey 1994 E420 beautiful car,gone to new home 1994 E320 Smoke an Parchment.Beautiful car 
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#2
03-21-2006, 04:36 PM
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I would suspect that a bad vacuum pump would show other symptoms--such as an inability to turn the engine off with the key, and door locks not functioning properly.
Have you tried the pedal with the engine off? If you have strong pedal that way, I would suspect the brake booster. If you try it with the engine off and the pedal still goes to the floor, I would recommend re-bleeding. I use a one-man vacuum pump bleeder, and it works well. Be sure to keep both chambers in the master cylinder full. And of course, there is always the possibility that the master cylinder itself is faulty. Good luck. Joe B.
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#3
03-21-2006, 04:45 PM
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Sounds like the classic problem of running the back half of the brake fluid reservoir dry. You have to fill the reservoir practically to the neck before fluid overflows from the front portion (which supplies only the front brakes) to the rear portion. Look at the reservoir with a strong light and you can see whether the rear portion of the reservoir is dry.
- JimY
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#4
03-21-2006, 04:59 PM
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There will be a check valve that tries to hold built up vaccum in the booster. Sounds like it might be wide open. Will be located outside the booster. Seems like at full accumulated vaccum pressure brakes work well. High engine speed is making good vaccum. At lower speeds vaccum is low. Pump cannot develop enough. Or it is flowing back out of booster to equalise. Might also be from leakage of vaccum somewhere. To test the booster for small internal leak get hold of vaccum gauge with pump up feature. The booster should be able to hold constant vaccum. But first find out if the check valve in that line is closing. Nice if that is all it is. After problem is repaired check vaccum pressure at idle for factory specs. Chase down any additional leakages if any. If you own one of the 123s a vaccum testor is a good tool to own. The rebuild kit for the pump is cheap on the earlier models like you have as well but make sure you need it first. Check valve in vaccuim pump can be marginal as well but again with tester you can isolate whatever is causing the problem. A real good ideal is to check the previous information in the archives out as well.
Last edited by barry123400; 03-21-2006 at 05:25 PM.
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#5
03-21-2006, 06:14 PM
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Sounds to me like it may be both vaccum and low on fluid in the rear half of reservoir. That rear reservoir sneaked up me when I was bleeding the brakes the first time...check it first....although you should have way more than 5% braking if the front brakes are working good.
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Mark 1983 300TD Wagon Even a broken watch is right twice a day
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#6
03-21-2006, 06:17 PM
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Bleed the front and rear systems. Make sure the fluid in the rear of the brake reservoir is not empty.
__________________
1981 300D 147k 1998 VW Jetta Tdi 320k 2001 Dodge Ram 2500 141k 1979 300D 234k (sold) 1984 300D "Astor" 262k(sold) Mercedes How-To and Repair Pictorials I love the smell of diesel smoke in my hair 
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#7
03-21-2006, 07:34 PM
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Quote:
Its actually the opposite, the rear chamber is for the front brakes, while the front is for the rear brakes.
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-diesel is not just a fuel, its a way of life-  '15 GLK250 Bluetec 118k - mine - (OC-123,800) '17 Metris(VITO!) - 37k - wifes (OC-41k) '09 Sprinter 3500 Winnebago View - 62k (OC - 67k) '13 ML350 Bluetec - 95k - dad's (OC-98k) '01 SL500 - 103k(km) - dad's (OC-110,000km) '16 E400 4matic Sedan - 148k - Brothers (OC-155k)
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#8
03-22-2006, 10:14 AM
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Thank you VERY much. Great replys and most of them within one hour of posting. car will be home in a day or two, can't wait.Love these coupes and this one is beautiful.
On the brake problem i will bleed anyway but look close at the vacuum check valve on or before the booster. Power door locks work fine and motor turns off. Motor takes about 1.5 seconds to quit after turning key off. Well till i get it home no use guessing anymore. Again thanks Rich
__________________
88 300E White, sold 9-04 88 190E Black 2.3 sold 90 300CE White, Grey sold 12-24-04 92 400E White,Grey Sold 5-06 90 300CE 91K Signal Red,Black Sold 2007 1989 300CE Smoke silver,Tan Sold 1-08 1993 300E 3.2 M104 motor 1991 300CE White,Grey 1994 E420 beautiful car,gone to new home 1994 E320 Smoke an Parchment.Beautiful car
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