The Mystery of the Disappearing DOT4 - PeachParts Mercedes-Benz Forum

Hit Man X · #1 12-22-2009, 09:33 PM

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.

Thoughts

leathermang · #2 12-22-2009, 09:37 PM

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...

johnjzjz · #3 12-22-2009, 09:49 PM

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

tangofox007 · #4 12-22-2009, 09:54 PM

Quote:

Originally Posted by johnjzjz

would you use a recap tire?

They are good enough for the airlines.

johnjzjz · #5 12-22-2009, 10:10 PM

Quote:

Originally Posted by tangofox007

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

Hit Man X · #6 12-22-2009, 10:11 PM

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.

funola · #7 12-22-2009, 10:20 PM

Have you looked inside the booster? It could hold more than 2 quarts.

Hit Man X · #8 12-22-2009, 10:22 PM

Quote:

Originally Posted by funola

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.

t walgamuth · #9 12-23-2009, 06:56 AM

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.

swheele2 · #10 12-23-2009, 08:46 AM

Quote:

Originally Posted by t walgamuth

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.

funola · #11 12-23-2009, 10:41 AM

Quote:

Originally Posted by swheele2

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!

leathermang · #12 12-23-2009, 09:04 AM

Quote:

Originally Posted by t walgamuth

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....

Hit Man X · #13 12-23-2009, 02:12 PM

Damnit, so this could kill my booster?

Looks like I will get the reman on order to swap out and do it the first of next week.

FWIW, my pedal feel is fine.

4x4_Welder · #14 12-23-2009, 08:47 PM

Chances are that the DOT4, combined with the lack of driving, is what's killing MCs. DOT4 is extremely hygroscopic, much more so that DOT3, and that water causes corrosion in the bore. When the seals ride over this corrosion, they get damaged, and cause leakage.
If you don't rely on this car as a DD, then I would pull the master that's on it, and rebuild it myself. Pull it, disassemble, and check the bore the whole length with a T gauge. If it's within specs after being honed, then get a kit for it, reassemble, and be done with it. If you still use DOT4, make sure to get a compatible seal kit. Replace your grommets and fill cap seal at this time as well.

Hit Man X · #15 12-23-2009, 09:01 PM

Quote:

Originally Posted by 4x4_Welder

Chances are that the DOT4, combined with the lack of driving, is what's killing MCs. DOT4 is extremely hygroscopic, much more so that DOT3, and that water causes corrosion in the bore. When the seals ride over this corrosion, they get damaged, and cause leakage.
If you don't rely on this car as a DD, then I would pull the master that's on it, and rebuild it myself. Pull it, disassemble, and check the bore the whole length with a T gauge. If it's within specs after being honed, then get a kit for it, reassemble, and be done with it. If you still use DOT4, make sure to get a compatible seal kit. Replace your grommets and fill cap seal at this time as well.

Hmmm, well the car is driven at least a few times a week. Brakes are flushed in the spring due to the absorption of water...

Interesting observation that the DOT4 is breaking it down, but would make sense I suppose as it is a US supplier that remans the units probably only specs the seals for DOT3 as almost all US stuff uses it.

Appears that my best bet is to return for refund and install a new one from FTE.

At least the oil change today went easy.