Spongy brakes - 1991 300E - PeachParts Mercedes-Benz Forum

mbzr4ever · #1 10-13-2015, 10:18 PM

Took an alternate route to work this morning, one that has a very steep, long hill down about 2.5 to 3 miles in length, from about 1000 ft to sea level.

I used the brakes a lot, and then they began to feel spongy, like I had to pump them for them to work.

After some driving on level ground, the brakes appeared to work normally again.

I looked for some leaks, but did not see any around the grommets or around the reservoir.

Is this a master cylinder failure? Or air in the lines or ????

Frank Reiner · #2 10-13-2015, 11:10 PM

Your description matches what is often called "boiling the brake fluid". Brake fluid, with the exception of DOT 5, absorbs water from the air. That water becomes dispersed throughout the fluid, and when the calipers become hot enough, the water vaporizes. The water vapor is quite compressible, leading to the feel of a spongy pedal. When the calipers cool again, the water vapor returns to the liquid phase, and the pedal is once again firm.

mbzr4ever · #3 10-13-2015, 11:36 PM

Thanks, Frank for that info.
When going down that steep hill, it did smell like the brakes were overheating.

I thought I was using DOT 5? At least the container says so, but maybe it was too old?
Does the fact the weather here has been exceptionally humid/rainy the past few months give the brake fluid more water to absorb?

Frank Reiner · #4 10-13-2015, 11:49 PM

Quote:

Originally Posted by mbzr4ever

Thanks, Frank for that info.
When going down that steep hill, it did feel like I was working the brakes.

I thought I was using DOT 5?
Does the fact the weather here has been exceptionally humid/rainy the past few months give the brake fluid more water to absorb?

The past few months? Is not that the normal state of affairs in the land of Dole? A dry day appearing with the same frequency as in London!?

The usual brake fluids are DOT 3, & DOT 4. Absorption of water occurs over a period of years.

mbzr4ever · #5 10-14-2015, 02:37 AM

Quote:

Originally Posted by Frank Reiner

The past few months? Is not that the normal state of affairs in the land of Dole? A dry day appearing with the same frequency as in London!?

The usual brake fluids are DOT 3, & DOT 4. Absorption of water occurs over a period of years.

Quote:

The past few months? Is not that the normal state of affairs in the land of Dole? A dry day appearing with the same frequency as in London!?

Not on this leeward side of the island, LOL - except for this el nino summer!

Quote:

The usual brake fluids are DOT 3, & DOT 4. Absorption of water occurs over a period of years.

I'll check the container again - does the brake fluid need to be changed because I fried it?

Skippy · #6 10-14-2015, 08:56 AM

We also call that situation "brake fade". I wouldn't call a fluid change essential just because you got a bit of brake fade, but it certainly wouldn't hurt. It would also be a great idea if it hasn't been changed in the last year or two.

Frank Reiner · #7 10-14-2015, 10:11 AM

Quote:

Originally Posted by mbzr4ever

I'll check the container again - does the brake fluid need to be changed because I fried it?

That would certainly be my recommendation; if the fluid temp rose to the point that vaporization occurred, the system should be flushed.

The "fade" to which Skippy referred is caused by the temperature of the pad friction material rising to the point that the coefficient of friction drops dramatically; the pedal can still be hard, but there will be almost no braking force. And it can also occur in conjunction with "fluid boiling".

mbzr4ever · #8 10-14-2015, 01:21 PM

Will study up on changing the fluid, thanks guys.

When driving downhill in my 240D or truck, I have the option of using a lower gear to help slow the car down.

I have heard different opinions if this is OK with an automatic transmission.

What say ye, oh - Mercedes forum gurus?

Frank Reiner · #9 10-14-2015, 08:07 PM

Generally, the use of lower gears in autoboxes is recommended by the car makers for descending steep grades; it certainly is advised by Mother Benz. Perhaps your car has the "B" notch to the right of L in the selector? "B" = Braking!

mbzr4ever · #10 10-15-2015, 02:04 AM

Quote:

Originally Posted by Frank Reiner

Generally, the use of lower gears in autoboxes is recommended by the car makers for descending steep grades; it certainly is advised by Mother Benz. Perhaps your car has the "B" notch to the right of L in the selector? "B" = Braking!

No, there is no "B" notch on my transmission selector.
I was warned not to use a lower gear, ie 1 or 2 on the automatic or it might
damage the transmission.

Stretch · #11 10-15-2015, 05:17 AM

Word of caution about DOT5 - I think it is unlikely this has been used in your brake system as it is not all that common to do so.

DOT5 is chemically different with DOT4 and DOT5.1 (just to make things more confusing). Do not mix DOT5 with a DOT4 system or vice versa. (Well if you do you need to be aware of what you are doing)

https://en.wikipedia.org/wiki/DOT_5

https://en.wikipedia.org/wiki/DOT_5.1

https://en.wikipedia.org/wiki/DOT_4

Don't think of the DOT numbers in terms of the bigger the number the better your gonads are going to look! Just use what was originally recommended for the system (DOT4) unless the change to silicone has really already been made.

Frank Reiner · #12 10-15-2015, 02:34 PM

Quote:

Originally Posted by mbzr4ever

No, there is no "B" notch on my transmission selector.
I was warned not to use a lower gear, ie 1 or 2 on the automatic or it might
damage the transmission.

That warning certainly did not come from Mother Benz; see attached pic of page from Owner's Manual (2nd Gen W126 V8 w/ 722.3 trans).

"3" reads:
Upshift to 3rd gear only. Suitable
for medium range up or downgrades.

"2" reads:
Upshift to 2nd gear only. For
driving in mountainous regions.
Since transmission will not shift
up further, this gear selection
will make use of the engine's
braking power.

mbzr4ever · #13 10-18-2015, 04:38 PM

Frank,

Pretty convincing instructions, to me.
So it is ok to downshift with an automatic.

An old retired MB mechanic used to tell me otherwise.
He said it was cheaper to replace the brakes vs replace a tranny.
BTW this is in an area where steep hills are abundant, and brakes are used heavily, daily.

EDIT: I had used DOT 4.

mbzr4ever · #14 02-28-2016, 12:37 PM

Tried to bleed the brakes this past week - now worse than ever.
Think I made a big mistake and now there is air in the acuator (?)... in any event, I'm gonna need a pressure tool to complete the job correctly.

(I even took it a local mechanic, but he won't touch it cause of the additional complexity entailed because it is a mb/abs/blah/blah...)

I have a low compression air pump (for air brushing) - can this be modified to push air through instead of buying another gadget?

Stretch · #15 02-28-2016, 12:54 PM

Buy another gadget. I've used one of those Gunson easy bleeds for years (https://www.youtube.com/watch?v=Lz8t6ZkVSQM) and have had little trouble with it unless the connections on the top of the reservoir leak then it is a pain...

...similar reservoir pressurising systems are available...

...but more recently I've been playing about with a vacuum system that sucks fluid through from the bleed screw. Mity vac seems to be the forum favourite but cheapo versions are abundant (which won't last as long but hey you gets what you pay for)