The Mystery of the Disappearing DOT4 - Page 2 - PeachParts Mercedes-Benz Forum

funola · #1 12-23-2009, 10:57 PM

Quote:

Originally Posted by 4x4_Welder

.....DOT4 is extremely hygroscopic, much more so that DOT3, and that water causes corrosion in the bore......

Can you provide more info on DOT-4 being more hygroscopic than DOT-3? I thought they are both glycol based and equally hygroscopic. The only difference between the 2 is DOT-4 has a higher boiling point.

Hit Man X · #2 12-23-2009, 11:01 PM

Quote:

Originally Posted by funola

Can you provide more info on DOT-4 being more hygroscopic than DOT-3? I thought they are both glycol based and equally hygroscopic. The only difference between the 2 is DOT-4 has a higher boiling point.

AFAIK it is what, 25-50°F difference in boiling point?

funola · #3 12-23-2009, 11:34 PM

Quote:
"From:

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. "

The Pontons is what, from the 50's? From the description above, it sounds like the booster design is very different than one from the W123. It even mentioned a "leather diaphram" and also a "vacuum line under the booster (above the pedals)".

I also had a mystery DOT-4 (than DOT-3 because it was cheaper) leak for a couple of months, with no external leaks. The mystery was solved when I pulled the MC and found it almost filled to the level of the MC. I sucked it all out with a syringe and stuffed Bounty towels in there to soak up what's left and put the used MC from my 240D parts car in there and the brakes has been working fine since. The vaccum line on the W123 is on top of the booster as I recall so will not suck all the brake fluid which has leaked in there, just whatever gets splashed up to the vac hose. I meant to check the booster with a Mightyvac to see if it has developed any leaks due to being half filled with brake fluid (DOT-4 then DOT-3), which has not caused a catastrophic failure of the booster yet, but haven't gotten around to it since the brakes has been working fine. Maybe I'll do it soon.

Jim Anderson · #4 12-24-2009, 12:06 AM

I've had that first seal start leaking oil inside the car onto the floor. Didn't notice it until the carpet got saturated and my feet started slipping off the pedals.

4x4_Welder · #5 12-24-2009, 12:22 AM

Quote:

Originally Posted by funola

Can you provide more info on DOT-4 being more hygroscopic than DOT-3? I thought they are both glycol based and equally hygroscopic. The only difference between the 2 is DOT-4 has a higher boiling point.

I dealt with it in school, but for the life of me I can't find the info now. I remember having a conversation about it with the instructor in the brakes class because my CB400 required DOT4. He told me that it has higher boiling points both wet and dry, but absorbs more water and risks more corrosion.

FWIW, there are DOT3 fluids out there that exceed the DOT4 requirements.

Hit Man X · #6 12-24-2009, 01:16 AM

I have been using the DOT4 as it is stamped on the brake fluid cap last time I have checked it.

FWIW, my other 126s ( and BMWs, trucks, the Volvo) have no issues with the same DOT4... I really just assume I have been receiving ****ty remans.

ck42 · #7 12-24-2009, 10:49 AM

funola:

Quote:

The Pontons is what, from the 50's? From the description above, it sounds like the booster design is very different than one from the W123. It even mentioned a "leather diaphram" and also a "vacuum line under the booster (above the pedals)".

There's still a seal that exist between the MC and the booster. If this seal is failing/failed, fluid WILL leak into the booster.

SirNik84 · #8 12-24-2009, 03:27 PM

You may be past this point, but the first thing I would to is a little trouble shooting. Have you tried removing the lines leaving the master cylender and installing plugs, and stepping on the brake peddle? If you are questioning the master cylender as a unit this will disable the rest of the system so you know for sure that the leak in internal. the peddle should not move because there is no where for the fluid to move to. If it moves at all then one of the seals in the master cylender is bad.

hope that helps, good luck

Hit Man X · #9 12-29-2009, 08:30 PM

NEW master cyl arrived today, will probably install this weekend.