Vaccuum Brakes on '68 280SE

[Mr Basil]

Hi,

The vacuum assisted brakes on the '68 280SE seem to have been reduced to manual only. I believe there are no leaks in the lines or slave cylinders. Also I hear a little whoosh or phssst when depressing the pedal. My Questions:

• Does the replacement of the MC mandate replacement of the vacuum booster?
• If not, how do I determine the faulty component?
• Is this something I should order at my local autoparts store (which is usually quick) or should I be looking on line?

Thanks,

Basil

[250 Coupe]

If yer up for a drive, I'll be putting up a fence this weekend and will probably need a break at some point.

I'm at 272nd and Pacific Highway South in Des Moines.

PM me if you are thinking of stopping by.

Michael

[Mark DiSilvestro]

Originally Posted by Mr Basil

Hi,

The vacuum assisted brakes on the '68 280SE seem to have been reduced to manual only. I believe there are no leaks in the lines or slave cylinders. Also I hear a little whoosh or phssst when depressing the pedal. My Questions:

• Does the replacement of the MC mandate replacement of the vacuum booster?
• If not, how do I determine the faulty component?
• Is this something I should order at my local autoparts store (which is usually quick) or should I be looking on line?

Thanks,

Basil

Brake master cylinder replacement doesn't automatically mandate a new power-brake booster except possibly if you had long-term fluid leakage from the master cylinder into the booster.
Did you replace your brake master cylinder, and find alot of brake fluid inside the booster? That could have damaged the booster's diaphragm or control valve. Leaks in the lines or calipers won't cause booster failure.

If your brake-pedal feels very hard and requires that you practically have to stand on the pedal with both feet to stop the car, your brake power assist has failed.
The 'whoosh' noise suggests the vacuum supply to your booster is still working.

There's an 'O' ring gasket between the master cylinder and booster that sometimes gets overlooked. Though, in my experience, if the 'O' ring is intact, it may still be useable.

I doubt a booster of this vintage would still be availible from your local autoparts store, but it doesn't hurt to ask.

Happy Motoring, Mark

[Mr Basil]

Hmmm.

So far, I have only observed a gradual deterioration of braking over about a two week period that seems to have leveled off in it's current state.

The pedal depresses without much pressure (with the little whoosh) getting gradually harder as it approaches the floor, yet never what I would call hard. I believe I will stop when I need to, but it feels wrong and takes longer. I am now driving my wife around since she won't drive it. I need to fix it soon! I hate to replace stuff that doesn't need replacing but I will have to do something pretty quick here because my current inefficiency is frustrating.

Basil

PS 250, probably won't make it to your place anytime soon since I'm about 70 miles on the other side of the narrows bridge and particularly slammed for time from now through summer, but will let you know if I have any errands that would bring me somewhat in your vicinity.

[Mark DiSilvestro]

Main symptom of a failed brake booster is a rock-hard pedal that barely moves and practically requires both feet to stop the car.
If the pedal "depresses without much pressure" and "approaches the floor", it sounds like air in the system or a fluid leak. If there's no external fluid leak, you could have an internal leak inside the master-cylinder, where the fluid bypasses the piston via a faulty seal, and returns to the reservoir.

Another thing to check that sometimes causes a mushy, funny-feeling pedal, is the brake pads wearing down so far that they hit the caliper spring-plates, which cocks them and prevents them from properly hitting the rotors.

Happy Motoring, Mark

[Mr Basil]

Thanks for the info, I hope you are right and it's just the MC.

Further observations: The whoosh sound is air expelled from the boot around the rod going into the booster from the pedal. When the car is not running the whoosh dissipates after a few complete pumps and a subtler, yet similar sound occurs, like fluid under pressure rushing through a hole.
I guess I'll order a new MC. The air escaping from the boot is the part I find most confusing.

More comments and ideas welcome!

Basil

[Mark DiSilvestro]

Originally Posted by Mr Basil

Thanks for the info, I hope you are right and it's just the MC.

Further observations: The whoosh sound is air expelled from the boot around the rod going into the booster from the pedal. When the car is not running the whoosh dissipates after a few complete pumps and a subtler, yet similar sound occurs, like fluid under pressure rushing through a hole.
I guess I'll order a new MC. The air escaping from the boot is the part I find most confusing.

More comments and ideas welcome!

Basil

With the engine running, a momentary 'whoosh' each time you step on the brake is normal brake booster operation, as are the few 'whooshes' after the engine is shut off. It's the sound of the pedal valve letting air into the booster to allow the vacuum diaphragm to do it's job. However, with the engine running, if the whooshing continues as long as the pedal is depressed, that is not normal and indicates a failed booster diaphragm.

Happy Motoring, Mark

[Mr Basil]

Thanks Mark!

Now I am much more confident that it's the MC. So My next Q is where's a good place to get one (or a rebuild kit) ?

I can see this problem would have been smaller if we only owned another SE for comparison...

Basil

[Mr Basil]

So much for confidence!

Actually I'm a fan of confidence, but the new MC doesn't seem to have improved things much. The vacuum line is pulling vacuum. The pedal feels great until you start the car. Then it gets soft!
I'm going to order a booster ($$) Tuesday unless somebody can think of something that will indicate another cause.

Thanks,

Basil

[Mike D]

The pedal getting "soft" or going down a bit when you start the car is normal. It is the function of the vacuum assist.

I'd check your calipers for rust or seizing. Common for older cars. The caliper pistons will form a ridge and when the brakes wear down the calipers require much more effort to squeeze the rotors.

Pull the wheels and check ALL the calipers by completely depressing them. The front seem to be the most prone to seizing.

If one of the calipers is seized it creates an expansion chamber which has to be pressurised BEFORE it's mate on the same line will work. You don't feel it in the pedal because the master cylinder is doing it's job but you do notice it in the decreased braking abilities.

[Mr Basil]

I will check the calipers before I order a booster.

I want to say that when the pedal gets soft when the engine is started, I mean it drops several inches before you get any brake action (!) which is not even close to how it should (and used to) be.

Thanks,

Basil

[Mike D]

If the pedal comes "up" on the second or third pump then double-check for air in the calipers.

You might find air in the left front caliper after a master cylinder change. It's the shortest line and any air which sneaks into the system usually gets pushed there.

[Scutch]

I got mine for $300 from this guy Will Samples. you can get rebuilds done on the West Coast for $160 - $300. I have never tried them.

Eddie

Filename: http://www.mbzponton.org/valueadded/other/ssimports.txtRevised: 03-nov-2005Created: 02-nov-2005S&S Imports / Dallas, TexasAll early Mercedes-Benz work, specializing in brakes, brake boosters, master cylinders, carburetors, keys (large collection of original key blanks), and more. Helpful technical support.S&S ImportsMr. Will Samples3303 Wendy LaneDallas, Texas 75214Tel. 214-826-5977FAX 214-826-5294ssimports@sbcglobal.net

[Mr Basil]

Happy Holidays Everyone!

I rotate through my various mechanical interests. Mercedes in the winter, vintage tractors in the spring, too much other work in summer and old trucks in the fall. I was just going through some of my old posts so I wouldn't be redundant, and found I never told you guys what was wrong with the brakes. One of the driver's side caliper pistons had a crack parallel to the brake disc. The crack would squeeze shut most of the time but sometimes it would lose a lot of fluid. I didn't see it until after I replaced the whole caliper because it was just a hairline crack, but it nearly drove me mad for awhile!

It is good to be back!

Basil

[JeffreyNMemphis]

Thanks for the update Basil...

This is the first time I saw the thread. We love a thrilling conclusion. Just like a movie where the good guy triumphs, the town is saved, and the would-be spinster finds true love.

Cheers!