Main.TrailIndexPage | Brake System

Brake System

Definition

The brakes (bremse) are used to slow and stop the car. A dual circuit Master Cylinder is used for safety. One circuit operates the front Disc Brakes, and the other circuit operates the rear Drum Brakes (230 SL) or disc brakes (250/280 SL) . The Hand Brake is used to hold the car when parked and to stop the car if the main hydraulic system fails.

Function

Pushing on the brake pedal increases the pressure of the brake fluid in the master cylinder. The brake Booster, or servo, uses engine vacuum to increase the pressure in the system. The increased hydraulic pressure travels through the Brake Lines & Hoses to the brake Caliper (for discs) or Wheel Cylinder (for drums). In the calipers the pressure extends the brake Pad to squeeze both sides of the rotor to slow the car. In drum brakes, the Wheel Cylinder expands to press a pair of semi-circular brake shoes against the inside of the brake drum.

Components

List all components here that comprise the section. These would typically be links branching off to new component pages.

Maintenance

The brake system is the most important safety system in a car. Therefore it is critical to maintain the system to keep it functioning correctly and to avoid failures. There are three components of the system that should be the focus of ongoing maintenance: brake fluid, brake pads (and shoes), and brake hoses.

Brake Fluid

Brake fluid, like any fluid, is non-compressible and is used to transfer the force on the brake pedal to squeeze the brake pads against the rotors to slow the car. Both air and water are detrimental to brake fluid. Air IS compressible and if there are air bubbles in the system, they will compress when pressure is applied and less force will be applied to the pads. If there is air in the system, the brakes are usually described as "spongy" and they must be bled to remove the air. Air can enter the system whenever a hydraulic fitting is opened, or when the brake fluid reservoir is emptied. See Bleeding brakes for more information.

Water in the system also causes problems. Traditional brake fluid is hydroscopic and will absorb moisture from the air over time. The first problem is that the water will rust the brake lines, pistons, and wheel cylinders from the inside. Water also lowers the boiling point of the fluid. When the brakes are applied repeatedly, or for long durations, the built up heat can cause the fluid to boil, bringing air out of suspension from the liquid and making the brakes spongy. The decrease in brake performance is called brake fade. To minimize the problem use good quality DOT 4 brake fluid and change the fluid completely every two years. Some manufacturers make two colors of the same fluid so you can tell when the old fluid is flushed out.

Silicone brake fluid was developed at the request of the U.S. Postal Department to reduce the maintenance costs in their huge fleet of vehicles. It does not absorb moisture from the air and therefore does not have to be changed as often. Many swear by it, but some say that the material that the older seals are made of don't absorb silicone brake fluid very well and it can cause leaks. See Brake Fluid for more information.

Brake Pads and Shoes

Brake pads and shoes are the main wear component in the brake system. They should be checked on a regular basis to monitor the wear so they can be replaced before they reach their minimum thickness. But they should also be checked for uneven wear, which could indicate a stuck piston. Disc brake are self adjusting, but drum brakes (including the drum handbrake on the 250/280 SL) must be adjusted to accommodate wear. See Drum Brakes and Brake Pads for more information.

Brake Hoses

The flexible brake hoses that allow each wheel to move in relation to the car are a fragile link in the brake system. They are subject to fatigue from constant movement, damage from road debris, and internal deterioration. Fatigue and damage can cause leaks. Internal deterioration can block the fluid flow either to or from the wheel, the first not allowing any braking, the second locking the pads to the rotor, both very dangerous. The hoses should be checked on a regular basis for leaks and damage and felt along their length for soft spots that indicates internal damage. It's good insurance to replace them on a newly purchased car and about every five years thereafter. See Brake Lines & Hoses for more information.

Old Yahoo content

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On my 1965 230SL, the brakelight switch, including the surface that the pedal rests against, appears to be made out of plastic or nylon. No metal to metal contact, so very little noise. It appears to be the original switch. Do you think that the nylon part clips onto the nut that holds the switch in place? Maybe we are all missing it. I had a look on my EPC CD and there is some sort of part shown there. It is called 'pedal end stop' I can't say if that is perhaps a plastic or rubber part or even if I have it or not. I ordered one to see what it is. Ok, here is the bad news. As I mentioned earlier, my EPC shows a 'pedal end stop'. I don't know what it is made of or if we need that one but I did try to order one from MB here. Reply from MB: No longer available and no replacement either. Part no. 113 292 0041. As said, I don't even know if this is what we need to stop the noise.

When I disassembled my brake light switch the nylon ring broke apart and therefore off of the nut assembly. I looked far and wide for a replacement but the part is not available nor does any parts man working in MB have any idea what we are talking about. (Ray at Star knows exactly what we need but he assures me that this part is not available either.) So, what do you do if you are missing the nylon?? I took a piece of reinforced sheet rubber and cut it out to be the same size as the flat surface of the brake switch nut. Then I contact cemented it to the nut. To date, (six months) the rubber on the nut has worked like a charm and provides the slightly soft surface for the brake pedal to rest on and impact when you let your foot off of the pedal. Be sure to use a very firm rubber that will not easily deform under the constant spring pressure of the brake pedal.

Tom Hanson: the Mercedes-Benz Classic Center confirmed todat that this part isn't available anymore. By looking at it in the parts illustration, it seems as if it would be easy to fabricate.

I have solved the mystery of the brake pedal noise. It is easy to do. I used a piece of 5/8" = 16mm radiator tubing cut just a little longer than the nut is and pushed it on there. I had to do it 2 times because my first tubing was not long enough to stop the noise when quickly letting the pedal snap back. Also for consideration is that the light switch must still work properly.


Michael Salemi: I think that my car is "diving" under braking a little more then it should, but then again maybe that's normal. It does not seem normal to me based on experience with other cars. In the past on other cars, this would point to the proportioning valve, or as MB calls it, the Brake Force Regulator. Of course, they give a procedure for testing it which is totally impractical (involving major hydraulic pressure testing); so my guess is it either works or you replace it. Problem is I don't know of any way to practically test it "in the car". Aside from the diving, what other symptoms might I have? I'd hate to live with a bad one, and equally dislike installing a new one with no change in braking performance! If everyone has noticable diving under light to moderate braking, then I'm normal. If not, I'm going shopping for a new regulator...

Walter: You might benefit from an 'anti-dive' support member for the front suspension. Maybe that is worn out and is not up to its task anymore. You don't want ot mess with the proprtioning system especially if it involves the front brakes. Those take up about 75% of the braking force to stop the car, the rears are merely there to keep the front or rear in line. There is an equaliser in the back for the right left balance and that's it. If the front dives or dips slightly under moderate to severe braking that is normal, but under light braking, then something is worn out.

My understanding is that the proportioning valve's job is to divide the brake pressure, between the front and rear sets, in such a way that neither set will lock up it's wheels significantly before the other. That would have nothing to do with "diving", which is probably more a function of spring strength. Check out the recent posts about the variable-rate springs, and "diving".

The first thing to check is that the rear brakes are properly adjusted. The brakes on the W113 are not self-adjusting like most current cars. And if you do not adjust them often you will get an extra amount of distance that the rear shoes must travel in order to make contact with the drums. Thus the force on the front shoes will be much faster than the rear.

Cees: compared to modern cars, my W113 did seem to dive more than I would have excpected when I first got the car thre years ago. Then I read about the W113 and found this to be normal. Last weekend for the first time ever I drove someone else's 280SL, Tom Sargeant's to be precise, and he has the progressive rate springs and his car stays nice and level during firm stops - like a modern car. Then I started driving my car again since last weekend, and I was noticing that, now that I am used to it not being a modern car, how my car also does not really dive when braking moderate to even hard. As you say, it is hard to say anything about your car without driving it, but it does sound to me like yours dives more than normal. Perhaps Walter can post a bit more info on the anti-dive set-up he refers to in his post. Where is it / what are the components?

From my experience I always felt the frontend squat to be more than I cared for. However it is much worse when the rear brakes aren't operating or are working poorly. You might check the Hydraulic Pressure at each wheel. Should be easy in your case since you recently replaced all the connecting hoses and calipers and they won't be too rusty.

Bernt Damm: They always dive somewhat but if it is too much, maybe it is the shocks and springs. The brake booster either works or doesn't. No adjustments are made to it. If it fails, braking will be very hard. The diving was already complained about in the original road tests as it makes the car somewhat unstable because of the short wheelbase.

Pete Lesler: the brake force regulator only regulates pressure to the rear wheels to prevent lockup. If you disconnect it or connect it backwards, you typically experience the right rear wheel locking up under hard braking. Brake dive is more a factor of the suspension geometry of the front and rear suspension. Hard to get rid of it.

Unknown: The suspension of the W113 does not have an anti-dive feature, unlike the later MB models like the W116 or W107. To my mind it actually does have a detrimental effect on the handling. Essentially, as the rear lifts up, the suspension unloads and the wheels cant inward, effectively decreasing the rear track. Isn't that what was reported by contemporary tests as "tucking in the rear wheels"? Just my (non-mechanical engineering) opinion.
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