65mph & No Brakes (w108) - PeachParts Mercedes-Benz Forum

SoCalBomb · #1 09-05-2003, 03:56 PM

Yeah, it got pretty scary when all of a sudden I had completly no brakes and I was moving at about 65MPH. Seconds before the brake failure, I noticed gray smoke coming from the front wheel wells. There was none/vey little from under the hood. Although, I was still doing 65MPH, the smoke was still easily seen, so it must of been alot of smoke.

After I managed get the car off the freeway, using a combination of low gears and the parking break, I looked the car over. nothing brake fluid leaking, plenty of brake fluid in the little tanks above the master cylinder. The only miss was that the cap of the two brake fluid tanks was soaked with brake fluid, but not enough to make the brake fluid level go below min. I waited and waited hoping that the brakes would have to cool off, and that would ragain that pressure. But, that didn't work.

So, what could have happened. The brake failure was all-a-sudden. No warning, no sounds, just no brakes.

The model is a '69 280S (w108). I've had it for about a month.

jas2wa · #2 09-05-2003, 04:05 PM

That's pretty scary- glad you're okay.

Did you peek underneath to see if you lost a brake line or something up front? I suppose you'd see more fluid if so, but maybe it would only come out while activating the system. If the inside of any of your tires are covered in fluid, then you certainly lost a brake line.

Reminds me of the time in my old 70 280SE when, after kicking it down a gear to pass, the throttle linkage got stuck fully open. That was an exciting few minutes worth of driving... But losing all brakes is certainly scarier. Good luck-

JAS

csnow · #3 09-05-2003, 04:13 PM

No brakes at all, or marginal brakes?
I believe even your car has at least 2 'channels', meaning the rear brakes should do something even if the front channel goes out.

The smoke makes me suspect that one of the calipers stuck, and overheated, which in turn causes the brake fluid to boil, and the front channel to fail.

Does the pedal offer any resistance now?

Nice car, BTW!

Once had a caliper seize on a 450se. Plenty of smoke.
I was able to limp home with the caliper removed, and that line plugged with big screw and a hose clamp. Worked, but do not try this at home! I'm older and more risk-averse now...

NormanB · #4 09-05-2003, 04:17 PM

First - what a beautiful car, so glad you were able to bring it to rest without further incident.

You say you have no pressure when pedal depressed yet there is no loss of fluid from the brake resevoir - that is confusing.

I say right now I have no experience of your particular model but here goes.

If you are not developing pressure and without loss of fluid I would guess your master cylinder is shot or at least one of the internal seals.

But then there is the smoke - but no loss of brake fluid - must have been something else then OR did your brakes overheat and you have 'boiled' moisture in the brake fluid leaving a compressible oil/vapour mix that has compromised hydraulic efficiency - hell I don't know - the smoke business is confusing.

In the first instance I would buy a quart or so of brake fluid and run that through the system bleeding each line in turn (For routine do a search here loads of advice). If that does not restore system pressure you are going to have to delve deeper.

After a failure like that I would want to have a good look at master cylinder and each wheel caliper/piston/cylinder arrangement.

One thing for sure - you have had a lucky escape maybe you need to pile some dough into the braking system!!

Good luck

NormanB

SoCalBomb · #5 09-05-2003, 05:18 PM

yes, totally no brakes.

The brake pedal has some resistence, but only when the car is off. When the car is on, the only resistence seems to be from the friction in the pedal.

Pumping the brakes makes no difference.

I ordered a master cylinder, I'll install it ASAP.

I'll keep u guys posted.

JimSmith · #6 09-05-2003, 06:28 PM

Mike,

It sounds very much like a master cylinder failure. Once the seals fail, brake fluid gets behind the piston and it will not come back to its normal "off" positon. This causes the brakes to stay on, which causes them to overheat as you experienced. The net effect is for the pressure to build in the system, which makes the brakes burn hotter and faster, and so on until the pads wear down enough to reduce the contact pressure on the disc. Pumping the brakes has no real effect as the pistion is not coming back in the master cylinder. You are likely only exercising the mechanical linkage and pedal return spring.

Given the high heat and extreme stress of the event, the rubber seals in the calipers are probably shot too. I would also think the discs are probably no longer the same hardness they were from the factory. So, I would replace the caliper seals too and give the discs a good examination. I would also take a look at the grease pack in the wheel bearings.

The rubber seals are an integral part of the caliper function. If you look inside the caliper there is no return spring to pull the pad back from the disc when you take your foot off the pedal. And by design the master cylinder piston draws fluid to fill the void as it moves back from the brake fluid reservoir so it does not "suck" the piston back into the cylinder.

In actuality there is very little brake fluid flow when you apply the brakes - the fluid merely serves to transmit the hydraulic pressure, and other than the swelling of hoses and other pressure containing parts as the pressure rises, there is no significant flow of fluid (which is good or the rate at which the brakes responded would be based on the length and configuration of the flow path - which would be really dangerous as each wheel responded when it felt like it).

The return force to pull the pads away from the disc is provided by the seal. The seal is engineered to have a high coefficient of static friction with the piston, so when you push on the pedal and the piston moves out of the cylinder bore slightly to squeeze the pad against the disc, this seal (a square cross section "O"-ring) deforms. When youlet off the pressure, the seal relaxes and pulls the piston back slightly, unloading the pads from their contact with the disc. The actual travel is very slight.

If you have aged the seals they will no longer perform this function, and they may even leak. If they stop pulling the pistion back, you will burn up another set of pads as the pads will stay in hard contact with the disc. So, I would examine all the elements of the system to make sure they are ok. I am not sure about the availability of seal replacement kits for your car, but they are available for most and cost a few dollars.

The job is not particularly difficult, but you need a spot to clean your calipers good before reassembly. Cleanliness is vital when these go back together. And you need some patience as the external rubber boot (a new one is typically part of the kit) can be tricky to get on. I have done this recently and relearned to swear in languages I thought I forgot. The key is to be patient and it helps to do the job with the piston aligned to the cylinder bore in the vertical position, with the boot on the piston, but dangling off the end of the piston going into the cylinder bore first. Use only brake fluid to rinse, lube and rinse and lube the parts. Good luck, and I hope this helps, Jim

SoCalBomb · #7 09-05-2003, 07:05 PM

Thanks for all the deep thought into the issue, JimSmith. Good thing I have a break form school, so I can tackle this brake problem. As soon as I get these brakes fixed, I can't wait to sell the car.

yeldogt · #8 09-06-2003, 12:48 AM

JimSmith has it outlined

I have to say that the brakes on the 108s have been great almost no problems.

the master muts be the cause -- the system is so simple that it must be it -- let us know.

Also must say the car looks very nice - the white 108 is just a great Mercedes.

I will also say that a total failure of the system is very rare -- one must remember that this system was way ahead of its time. I also would say that rebuild is easy in this car -- the calipers were designed to be rebuilt.

yeldogt · #9 09-06-2003, 12:59 AM

I just reread the post -- SELL IT.

Why do that -- This thing can become a member of the family.

I spent all of my school days driving 108s -- the first one was not as nice as the one you have now.

They really are great cars -- I know that this brake thing is not fun -- but it is very very rare -- I have not ever seen it in a 108 MB -- that is why I would like to know what the outcome is.

Once you get them setup they are great -- you must change the oil and you must lube it - other than that it is very forgiving.

If you do sell it PM me

SoCalBomb · #10 09-06-2003, 01:14 AM

yeldogt,

I love the car, the whole brake issue is only a set back to me. The main problem I have with the car is the 10-12MPG. I'm a full time student/ work part time. I'm struggling to pay from my rising tution, and simply can't afford the luxury of owning a gas guzzler, no matter how much I enjoy. If the car did atleast 18MPG, I would consider keeping it, but currently it's killing me.

So, don't get me wrong the W108 is a great car, but simply doesn't fit my daily driver needs. And I can't afford to have it as a car on the side. So... I'm sorta forced to have to let it go. Hey, but if you wish to take it off my hands, please be my guest. Atleast I'll know it will be going into good hands.

New Developments:
I tried to pump the air and whatever else out of the brake system. The only place that reacted to the pumping was the right front. Nothing came out of the rest. I also pumped the resivours. Now I have a little brake power. My plan now is to drive it (easy) to a MB shop about 15 miles away, during the middle of the night. This way there will be less traffic for me to worry about. The MB tech at the shop is a family friend, and is confident he can fix the problem, with all his pro tools. I'll write back, as soon as I can, with what I learn.

kennysin · #11 09-06-2003, 01:45 AM

I have often wondered about the durability of the master pump and whether there is a recommendation for a service or replacement interval.

Most of the forum members would have cars at least 20 years old, and critical safety components with rubber seals in them surely will fail at some time. Although I regularly inspect brake pads and replace the brake fluids yearly, the issue of the master cylinder is usually ignored. The several times I broached it at my workshop, the argument is that failure is rare, and even in that event, the dual circuit setup preserves safety.

From your experience, this does not appear to be the case. Does anyone routinely include the master cylinder in the regular service?

SoCalBomb · #12 09-06-2003, 02:12 AM

It's smart that its a double circuit, but i'm sure there are classic MB drivers from this forum, using their second seal in the master cylinder. Seal kits or new/rebuilt master cylinders, are penny's in comparison to a costly accident, as a cause of a brake failure. I was lucky to get away from my experience, but had the driving conditon been different....I may not be writing right now.

ESVIC · #13 09-06-2003, 02:45 AM

It's a good thing your okay.....

Quote:

Originally posted by SoCalBomb
Yeah, it got pretty scary when all of a sudden I had completly no brakes and I was moving at about 65MPH. Seconds before the brake failure, I noticed gray smoke coming from the front wheel wells. There was none/vey little from under the hood. Although, I was still doing 65MPH, the smoke was still easily seen, so it must of been alot of smoke.

After I managed get the car off the freeway, using a combination of low gears and the parking break, I looked the car over. nothing brake fluid leaking, plenty of brake fluid in the little tanks above the master cylinder. The only miss was that the cap of the two brake fluid tanks was soaked with brake fluid, but not enough to make the brake fluid level go below min. I waited and waited hoping that the brakes would have to cool off, and that would ragain that pressure. But, that didn't work.

So, what could have happened. The brake failure was all-a-sudden. No warning, no sounds, just no brakes.

The model is a '69 280S (w108). I've had it for about a month.

SoCalBomb · #14 09-06-2003, 08:22 PM

New Developments:

I along with the mechanic tied bleeding the brakes again. The only one that blead anything, was the right front. Than we used an air pressure hose and pump to try to unclog the lines. All of them cleared up, except for the rear right. Eventually we took of the thick rubber brake hose that runs to the break, and cleared it out with copper wire. After intsalling it back, that side blead. We drained out the old nasty brake fluid and replaced it with new brake fluid.

The mechanic ordered 4 of those brakes lines, since the rest of them are probably in bad shape. They should come in Monday. I picked up the master cylinder today, and hope to get it installed this weekened.

One Problem Though....

On my way back from the mechanic, my brakes worked well. Except for the fact that they seemed to overheat, anjd smoke. I'm assuming there is too much break pressure in the brake system, even when not braking. While I waited for the brakes to cool off, I called the mechanic, and he said that installing the new master cylinder will solve this problem.

Does everything he said add up? Any theories as to what the problem maybe? Any questions?

WANT '71 280SEL · #15 09-06-2003, 09:41 PM

Maybe that's why your getting bad mpg's? Just kidding but I have heard people here talk of losing efficiency when brakes are wacked.
David