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  #1  
Old 12-22-2009, 09:33 PM
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Angry The Mystery of the Disappearing DOT4

Once again, I have disappearing brake fluid in my SD. I guess it has been about two, maybe three years since I replaced the MC last.

No calipers are leaking, hoses are new within the past five to six years, main lines are not rusted that I can see (been in Texas since new so the likelihood of that would be very, very remote). My only idea is back to the damned master cylinder which I have replaced twice already, previously it was sucking the brake fluid through the booster into the crank case!

The previous one that leaked internally into the booster had no external leak like I have seen before.

Anyone having issues with reman MCs? I may just scoop up a used one for $10 from a yard here with an '85 SD (ABS) and test my luck before buying new and having my money refunded for the reman.



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  #2  
Old 12-22-2009, 09:37 PM
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UH,,,, I would never use a used MC on a car.
While it is possible to get a new part which was manufactured improperly... the chances are so much smaller than of getting a bad one in the junk yard ...and the cost so low on new ones... that on brakes it does not compute to me...
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  #3  
Old 12-22-2009, 09:49 PM
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someplace you have to draw a line in the sand

would you use a recap tire -- new master is a good idea rebuilt caliper is really ok - jz
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  #4  
Old 12-22-2009, 09:54 PM
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Quote:
Originally Posted by johnjzjz View Post

would you use a recap tire?
They are good enough for the airlines.
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  #5  
Old 12-22-2009, 10:10 PM
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Quote:
Originally Posted by tangofox007 View Post
They are good enough for the airlines.

then Y are they landing on all the major HWYs in the USA rubber scraps are everyplace

wait may its just jersey that happens -- jz
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  #6  
Old 12-22-2009, 10:11 PM
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Thumbs up

Points taken. I believe two failures in less than five years to be unacceptable, it as not as if I had put another 150,000 miles on this car in that time frame... I would be surprised if I have put 5,000 on there. I will have to see if the auto store carries an alternate brand for the reman units or I will simply have my money refunded and purchase new.

If my MC is not leaking, where the hell could the DOT4 be going?

The only leak on the ground is from my 722.3 Guess I am going to take each wheel off and have someone do normal pumping on the pedal to see if maybe I am losing some while driving.
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I'm not a doctor, but I'll have a look.

'85 300SD 244k
'87 300SDL 251k
'90 300SEL 325k
'91 560SEL 189k

Eight others from IH, BMW, GM, Ford, and Volvo.

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  #7  
Old 12-22-2009, 10:20 PM
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Have you looked inside the booster? It could hold more than 2 quarts.
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  #8  
Old 12-22-2009, 10:22 PM
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Exclamation

Quote:
Originally Posted by funola View Post
Have you looked inside the booster? It could hold more than 2 quarts.


Cannot say that I have at this point. The loss has just started again within the past month to six weeks.
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I'm not a doctor, but I'll have a look.

'85 300SD 244k
'87 300SDL 251k
'90 300SEL 325k
'91 560SEL 189k

Eight others from IH, BMW, GM, Ford, and Volvo.

Liberty will not descend to a people; a people must raise themselves to liberty.
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  #9  
Old 12-23-2009, 06:56 AM
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I wouldn't be afraid of putting on a used one. They are extremely long lived. OTOH it is probably your MC leaking into your booster.

If your booster goes bad you don't lose all braking, you just have to push harder.
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  #10  
Old 12-23-2009, 08:46 AM
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Quote:
Originally Posted by t walgamuth View Post
If your booster goes bad you don't lose all braking, you just have to push harder.
Or the booster ends up sticking because of the fluid and you get dragging brake calipers. On my 84 300D the PO had replaced a bad MC but didn't remove the spilled fluid from the booster. I had a periodic brake-dragging problem and when I finally pulled the MC I found the booster almost completely full of brake fluid.
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  #11  
Old 12-23-2009, 09:04 AM
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Quote:
Originally Posted by t walgamuth View Post
I wouldn't be afraid of putting on a used one. They are extremely long lived. OTOH it is probably your MC leaking into your booster. If your booster goes bad you don't lose all braking, you just have to push harder.
How old is the one being gotten out of the junk yard ?
You or anyone else does not know..
so I would be afraid of suggesting to someone else they put a used one on.

I think that the rate of failure is high also. Is there any chance that someone associated with the MC is not in alignment... ? the push rod bent or something ? I do think you are going to have to look in the booster to find any further answers....
I do think at some point of neglecting warning signs that the inside of the booster can get ' hydrauliced ' .... hydro locked... liquid on both sides of the diaphram or piston which locks it up....
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  #12  
Old 12-23-2009, 10:41 AM
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Quote:
Originally Posted by swheele2 View Post
Or the booster ends up sticking because of the fluid and you get dragging brake calipers. On my 84 300D the PO had replaced a bad MC but didn't remove the spilled fluid from the booster. I had a periodic brake-dragging problem and when I finally pulled the MC I found the booster almost completely full of brake fluid.
Is it possible for a booster filled with brake fluid to cause the brakes to drag? The booster has a rubber diaphram with a spring and works by vacuum to give power assist to the master cylinder. How would the brake fluid (in the booster) cause the booster to push on the master cylinder and put drag on the brakes? Please explain!
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  #13  
Old 12-23-2009, 11:06 AM
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From: http://www.mbzponton.org/valueadded/maintenance/brakes.htm


Quote:
Pontons With Brake Boosters (power assisted brakes):
Do you have brake fluid disappearing slowly and you cannot see where? The master cylinder can leak slowly and the fluid dries up before you notice it. Or, the fluid is being drawn into the brake booster (if fitted). Rapid loss of fluid through the booster usually gets burned in the engine and makes it look like you are fogging mosquitoes. To stop the loss, unplug the vacuum line to the booster. Get the fluid out of the booster immediately. Brake fluid will ruin the rubber diaphragm inside the booster. Remove the vacuum check valve and drain the brake fluid out of the booster. Better yet, remove the booster from the car, undo the J-hooks holding the can on the booster, remove the can, and wash the diaphragm with alcohol or lacquer thinner. If you have a leather diaphragm, wash as above and soak in mineral oil. Are you bleeding the brakes after a brake job where you changed everything but the booster and the brake pedal never gets hard and you still get bubbles out of one of the wheel cylinders? Then, the booster is probably leaking. Pull the vacuum line under the booster (above the pedals) and check if the vacuum line is wet. When installing newly relined brake shoes, cover the lining with masking tape to keep grease off. When screwing all pieces of the brakes back together, if you cannot use your fingers to get the nut, bolt, or bake line screwed in most of the way, then stop. Something is wrong. This is especially true of the fine pitch brake fittings and the bolt screwed into the backing plate, through which the long thin pin passes and is held by a cotter pin.
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  #14  
Old 12-23-2009, 11:27 AM
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Quote:
Originally Posted by funola View Post
Is it possible for a booster filled with brake fluid to cause the brakes to drag? The booster has a rubber diaphram with a spring and works by vacuum to give power assist to the master cylinder. How would the brake fluid (in the booster) cause the booster to push on the master cylinder and put drag on the brakes? Please explain!
Went to How Stuff Works and found this:
The booster has a diaphragm that works in two modes.
Mode 1 is where the brake pedal is not pushed. Equal vacuum is on both sides of the diaphragm and the spring returns the pedal to it's starting point.
Mode 2 is where the brake pedal is pushed. A valve is actuated that seals off vacuum and allows normal atmospheric pressure buildup behind the diaphragm providing extra force on the master cylinder shaft.

When the pedal is released, the valve seals off normal atmosphere and opens to vacuum returning unit to Mode 1.

This, I suspected, was the case but had to verify.

Now for your question. The fluid can only enter the booster through the rear seal of the MC. It would go into the MC side of the booster which would indicate to me that it should resist or slow movement of the diaphragm. The fluid might be able to get to the opposite side of the diaphragm during the repressurization after brake pedal release or if there is a rupture of the diaphragm.

I don't think that a little fluid in the booster would cause too many problems but alot would defeat it's purpose - making the brakes much harder to push. It might even prevent the spring from returning the pedal to it's proper position thereby causing dragging of the brakes. Not sure if the fluid would cause degradation of the diaphragm.

For final diagnosis: A vacuum check of the booster would be in order as well as inspection for brake fluid in it.
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  #15  
Old 12-23-2009, 11:44 AM
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Did you not read this?

Quote:
To stop the loss, unplug the vacuum line to the booster. Get the fluid out of the booster immediately. Brake fluid will ruin the rubber diaphragm inside the booster.
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