84' 300D - Soft Brakes To No Brakes... - PeachParts Mercedes-Benz Forum

l3ushwig · #1 11-19-2009, 04:38 PM

Hello all,

I have a 1984 300D, and im having a problem with the brakes... I have searched up on this and i seen a couple threads that seemed like the same problem im having but nothing really definitively the same...

Id like to say it just started maybe 3 days ago and has since got worse daily.

At the moment i can start the car and let it idle, put my foot on the brake and within maybe 5 seconds it will eventually hit the floor but slowly. When i take my foot off the brake the pedal comes right back as it should.

I have not lost any fluid, all drums were dry as of yesterday.

I can put the brake all the way to the floor and shut the car off and it will shut off right away.

The only time i have had a vacuum problem is when its cold out and i dont cycle them via turning on the heat, thats the only time my doors wont lock and the engine wont shut off right away.

Ive even unplugged a few different vacuum lines (not knowing which are which, probably not smart i know, but they were right near the brake system...) while the car was running and tried the brakes and there was no change in the time of the softness.

Any ideas? Maybe point me to a thread i might have missed?

Thanks much!

ck42 · #2 11-19-2009, 04:40 PM

Sounds like a problem with the master cylinder....

l3ushwig · #3 11-19-2009, 05:41 PM

I was thinking the same thing to from the threads ive read.

However the one tri vacuum connector with the white bulb thing thats close to the firewall had the connections sealed with what looks to be some black silicone from the previous owner.

Not sure if that plays a role in anything to do with it or not.

ck42 · #4 11-19-2009, 11:42 PM

Any vacuum leaks in the system will result in a firm'er brake pedel....NOT one that is soft and goes to the floor. You mention that the engine is shutting of, which is also a good sign that you're not dealing with a vacuum leak.

It really sounds like the seal in the master cylinder is shot. When you press the brake pedal all the way to the floor, either that brake fluid has to be pushed SOMEWHERE or if it's not, then it's staying in the master cylinder because it can't be pushed out (which is the job of the master cylinder, right?)

Granted, you can also get this sort of behavior if you have air in the lines too. You press the brakes and the fluid gets pushed out but then it just starts compressing the air that's in the lines....allowing the pedal to be pushed more.

Have you had any brake work done recently?

Ian White · #5 11-20-2009, 01:00 AM

Sound to me like a master cylinder internal problem, fluid being diverted around the intensifier piston. How old is the brake fluid? Are the brakes working correctly other than the spongeyness? Could also be air in the lines...

l3ushwig · #6 11-20-2009, 11:00 AM

First off thanks for the replys, thats probably the problem then, the fluid hasnt been changed since i last worked on the breaks which has to be at least a year or so ago if i remember correct.

Looks like ill have to do some digging around in the attic and barn for the master cylinder from the 85 wagon i gutted.

Ill also have to try and find some threads explaining the topic more, even then it should be an adventure!

Thanks again

stimpy451 · #7 11-20-2009, 11:10 AM

After the initial sink to the floor, if you pump the brakes again, does the pedal get hard?

I just had this happen when I replaced my clutch cylinders - somehow managed to get air in the brake lines. Bled them and they were fine.

lkchris · #8 11-20-2009, 01:13 PM

Quote:

Originally Posted by l3ushwig

Looks like ill have to do some digging around in the attic and barn for the master cylinder from the 85 wagon i gutted.

Your master cylinder failed basically because it's 25 years old.

Replacing it with one that's 24 years old and hasn't had its seals constantly lubricated for a while doesn't sound like much of a fix.

Suggest a rebuild or replacement with new.

blackestate · #9 11-23-2009, 10:06 AM

This just happened on my 77 300D, w123. Will a MC from a 83 sd (w126) fit?

l3ushwig · #10 12-23-2009, 10:45 AM

Alright well lets face it i put the pro in procrastination... I still have not done this, ive just been using another car to and from work.

The car has been sitting with a foot of snow on it for almost 3 weeks, I just took all the snow off of it and im planning to start it up today and take it around the block a few times.

And stimpy, yes they will get hard again after the repump.

I suppose i could try to bleed them even though i didnt replace anything prior to this happening...

And yes i do agree it would probably be best for a new one since the old one has been sitting for 3 or so years in the elements without anything in it.

Codifex Maximus · #11 12-23-2009, 10:58 AM

Quote:

Originally Posted by l3ushwig

And stimpy, yes they will get hard again after the repump.

I suppose i could try to bleed them even though i didnt replace anything prior to this happening...

I agree with the others about bleeding and MC replacement.

Ok, so the brakes get hard after you pump them. Do they stay hard or gradually get soft again?

If they stay hard then you don't need a Master Cylinder - just a good brake bleed.

If they get soft gradually, you will need to replace the Master Cylinder or Rebuild then bleed the brakes.

Reasoning: The Master Cylinder has two units in it - front and back. Each is a separate system. One will keep the brake pedal solid so... if the pedal gets hard when you pump and then gradually gets softer, both systems in the MC are bad indicating a replacement or rebuild of the MC.

Most hydraulic systems will exhibit similar symptoms.

Edit:
It is possible for one of the systems to be failing in the MC and the pedal still be hard. The brakes will work but only on either the front or the back. If you find you keep needing to refill one or the other of the MC fluid reservoirs, this is an indication that one of the systems is failing.

l3ushwig · #12 12-23-2009, 12:52 PM

Well Codifex looks like its going to need to be replaced, they were hard then went soft, repumped to hard and slowly went soft again.

Ill have to look into a new Master or a rebuild if they make them instead of using my old replacement.

Codifex Maximus · #13 12-23-2009, 01:57 PM

Quote:

Originally Posted by l3ushwig

Well Codifex looks like its going to need to be replaced, they were hard then went soft, repumped to hard and slowly went soft again.

Ill have to look into a new Master or a rebuild if they make them instead of using my old replacement.

Before you buy the replacement MC, try bleeding the brakes *one* last time fore and aft. Just to be sure.

Bleeding method I use:
Use an assistant to pump the brakes to hard then bleed for a sec. Pump back up. Repeat. Don't run out of fluid in the reservoir. Work your way from the furthest wheel from the MC back toward the MC during bleeding. Bleed till the fluid stream is clear with no bubbles.

If there is no *leakage* of brake fluid at the wheel cylinders or on the lines to the MC during brake application and the brakes are still spongy after bleeding then the MC is the your strongest candidate for replacement.

One last thought, check the rubber grommets that attach your brake fluid reservoir to the MC. They should not be degraded.

l3ushwig · #14 12-29-2009, 06:41 PM

Was finally able to bleed the brakes on the car today!...

Started at passenger front to rear then drivers rear to front, 4 "bleeds" per wheel.

When done it seemed like the problem had only got worse but when i took it out for a ride the brakes went back and fourth with a mix of options from actual brakes to a weird soft brake ending in a little brake to hard brakes... Kinda hard to explain...

Im thinking maybe try to bleed again? Either that or im just going to get another master and swap out the booster with the old one i have for the hell of it.

Thanks again for the input!

snookwhaler · #15 12-29-2009, 06:48 PM

Quote:

Originally Posted by l3ushwig

Was finally able to bleed the brakes on the car today!...

Started at passenger front to rear then drivers rear to front, 4 "bleeds" per wheel.

When done it seemed like the problem had only got worse but when i took it out for a ride the brakes went back and fourth with a mix of options from actual brakes to a weird soft brake ending in a little brake to hard brakes... Kinda hard to explain...

Im thinking maybe try to bleed again? Either that or im just going to get another master and swap out the booster with the old one i have for the hell of it.

Thanks again for the input!

Start from the bleed screw that is farthest from the MC and work your way towards the MC. You need to start at the passengers rear, then the drivers rear, then the passengers front and then the drivers front. Check the MC often to be sure it is not running out of fluid.

Also, be sure to push the pedal slowly as not to build up air bubbles in the MC. It is also bad to go "all the way to the floor" with an old MC. Hold up just short and come back up.

If you can find a pressure bleeder. It works much better. But, if you have a patient helper.... That is fine too. Just do it the old fashioned way.

I agree with post #5. It sounds like the thing is leaking internally (bypassing).