Soft brakes + minor fluid disappearance, does it = bad MC? - PeachParts Mercedes-Benz Forum

azitizz · #1 11-12-2010, 02:14 PM

I have an 85 300TDT and had replaced the brake lines and did a bleed about 9 or 10 months ago. Once in a blue moon I get some softness in the brakes but it firms up soon after.

Now however they are going to the floor (still functional) and are very soft. I do have some vacuum problems too. (clunky shifting, sometimes funny ignition behavior) But thats been going on for a wjile and is likely unrelated but Ive read somewhere else a seal can cause this in the brake booster.

Theres been a slight decrease in the fluid in the reservoir over the last 9 months. Its gone down from just above the max line to about halfway between min and max. Ive never been able to find leaking brake fluid coming from the calipers or lines... so I'm wondering if this would most likely be the Master cylinder?

Is there any way of finding out by a visual inspection of it?
Thanks all.

SD Blue · #2 11-12-2010, 02:29 PM

From what you have described, I would be very concerned about the brake fluid ending up in the master cylinder. If brake fluid is getting into the check valves for the vacuum pump, it will likely destroy them as well. Brake fluid is very corrosive and if it ends up in other parts of the engine it's not good. If you find it in the brake booster, I would inspect the vacuum pump as well.

toddyvol · #3 11-12-2010, 05:54 PM

I had the same problem. I was losing brake fluid from the rear of the MC. In my case it was visible and was not filling the booster with fluid.

I separated my MC from the booster and took a look inside the booster with a flashlight. Mine was dry as a bone. I've read many instances where boosters were almost full of brake fluid.

azitizz · #4 11-12-2010, 08:22 PM

Hmmm interresting. Well I hope I'm wrong with suspecting the leakage into the Brake booster. As I said it has only gone down slightly in 9 or 10 months so it wouldnt be much. However I dont know where its going.

If it is indeed the MC, is it easy to tell from a visual inspection? Will it leak with pressure being applied to the brake? How would I test it?

Any diagrams or tutorials online?

benhogan · #5 11-12-2010, 08:27 PM

sounds like a bad MC. brake fluid might be getting past the piston inside the MC hence the pedal to the floor part.

the slight decrease in brake fluid (over 9 months) might be just a secondary problem i.e. there is a leak but it is too faint to notice.

i think the main problem is a worn MC

azitizz · #6 11-12-2010, 08:29 PM

Any way to be sure? How would I inspect the MC?

By removal and visual inspection would it be obvious?

layback40 · #7 11-12-2010, 08:33 PM

the level drop may just be as the pads are consumed from normal use. The peddle problem is most likely the MC. Either rebuild or replace. Dont bother with a second hand one as if they have been left sitting around for a while the internal seals soon deteriorate.

benhogan · #8 11-12-2010, 08:39 PM

Quote:

Originally Posted by azitizz

Any way to be sure? How would I inspect the MC?

By removal and visual inspection would it be obvious?

there are only three 'places' where the problem can be.

-you have a break in the actual brake lines (not the case)
-you have a leak in one of the calipers (not likely, since you only lost a little bit of fluid over 9 months-a leak in the caliper would drain the fluid in a matter of hours)
-that leaves the MC as the culprit

MBeige · #9 11-13-2010, 01:38 AM

I noticed my brake reservoir would have less fluid in it after a few months. My mechanic noticed some leak under the brake booster (under MC). True enough, MC was leaking. No fluid in booster, however (thank goodness).

Got a rebuilt MC, installed it and bled on vehicle. Then bled individual calipers with fresh ATE DOT4.

Washed reservoir with an open bottle of DOT4 to get almost all of the old fluid out. Grommets on new MC were nice and fresh so installation was harder (mating reservoir w/ new MC).

My brakes are stronger now, less effort required to stop vehicle. However it did not feel soft, in fact it felt hard prior. Now it's softer (easier to depress). But does not go to the floor.

toddyvol · #10 11-13-2010, 07:42 AM

Quote:

Originally Posted by azitizz

Any way to be sure? How would I inspect the MC?

By removal and visual inspection would it be obvious?

Simply unbolt the MC from the booster and peak into the booster with a flashlight to check for fluid loss. It does sound like your MC is failing.

I bought a rebuilt MC and have been very pleased. I bled mine on the car with the help of a friend rather than on a bench.

tangofox007 · #11 11-13-2010, 08:31 AM

Quote:

Originally Posted by SD Blue

From what you have described, I would be very concerned about the brake fluid ending up in the master cylinder.

If the master cylinder wasn't supposed to contain brake fluid, there probably wouldn't be a brake fluid reservoir setting on the top of it!!!

azitizz · #12 11-13-2010, 03:09 PM

So I just took the MC off and sure enough... The brake booster is full of fluid... So what now? It doesnt look apparent how to take the booster cover off and clean it out.

Also now is the whole vacuum system in troubble? I noticed fluid in the booster vacuum line.

What to do??

MBeige · #13 11-13-2010, 03:41 PM

Replace the MC and the brake booster.

The booster will be okay once dried out but won't last long.

If the fluid got into the level of the main vacuum hose, double check ALL vacuum lines.

azitizz · #14 11-13-2010, 03:47 PM

What can I use to clean and dry the lines out? Alcohol? Is there something that will dissolve and dry out brake fluid that I can use?

Why wouldn't the brake booster last? Because of the seals I Imagine?