What's the mechanism?

i also check and see if the vacum pump is working and see if you are get vacum at the brake booster

brian, not sure what you mean my mechanism. after thinking about it i am about convinced that the long pedal with pad wear theory might be in my mind only. i will try to check it sometime but it doesnt seem like a very high priority.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Since brakes are a fluid/hydrolic system, the pads/pistons in the caliper move to press against the rotors based on how much pad and rotor there is left, when you release the pedal and your pads are 90% worn and maybe half the rotor life is left, the pedal will travel no further as the pistons do not move back to their "with new pads/rotor" position. They simply move away enough to not apply any force to the rotor, which is extremely close to it. Thus when you press the pedal again the same amount of fluid force/pedal distance is used if the pads are 10% as when they are 100% as the pistons are moving the same exact amount of distance they did to begin with in order to compress on the rotors. You will get increased pedal travel/soft pedal if there are ballooning flex lines or other such problems but the wear of the rotors and pads won't make a difference. When I replaced my front pads/rotors the pads had about maybe 15% left and rotors maybe 30% but they were very warped so I just changed all of it. Afterwards the warp-vibration was gone but pedal travel/feel was exactly the same. Same for the '83 when we did the front brakes, rotors were deeply grooved and very little pad left (maybe 5%) after both new...same pedal travel, but the brakes did have a bit better feel/grip.

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Yeah it could be that- sometimes the engine doesn't shut off at all, othertimes it shuts off slowly, occasionally it's perfectly OK. Usually, I have to lock all the doors inside before shutting off- that usually works. Also, the central locking has stopped working altogether- I have to lock all doors and trunk manually. This certainly would account for greater pedal effort wouldn't it. It's pretty clear that the vacuum problem is in the locking system- something I don't care about anyway. I think I'll just isolate and disable it if I ever get the time

Dave

I'm in agreement with this.

So, I was curious to see what mechanism causes more pedal travel on worn pads. I can't come up with any.

Are you guys serious?
As the pads wear the caliper has to travel farther to make contact with the pads.
You never have to push the caliper back in to get the new pads in?

As the caliper is traveling farter to make cantact with the worn pad how do you make up the extra volume?
Doesn't it make sense that would lower your fluid level?

Danny

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Dannym,

I don't think anyone is questioning that the fluid level should decrease as the pads wear...it should. What is being questioned is the pedal travel and effort....they should not increase, given that the fluid level is replenished.

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AHHH, ok my bad.

That's a relief. I thought you people finally lost it.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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Pedal effort went up on my 300D. Turned out to be failing rears. Replaced claipers and rotors in back and all is well. Rotors were thin and the pistons were sticking in the calipers. Did not cost much, less than 200 for the parts and I did the work myself.

I'm happy to report I replaced pads and rotors, and everything's fine - thanks all for the good advice. By the way, I find the pedal effort much easier now- without any vacuum repairs. I think I can explain that from what the mechanic told me- the driver's side calipre was partly siezed, and he had to do a bit of work to get one piston retracted. Even with that, he only charged me for 1.5 hours.

I'm actually very surprised at how reasonable the cost is for these "routine" procedures on such an old car as mine. The set of pads and 2 rotors only cost $90 CDN, making this the least expensive brake job on any car I've owned. I'm finding the same on such things as oil and fuel filters- very reasonable costs and readily available.

Dave
1976 300D

Guys,

I just found this thread. What was overlooked in all of the discussions about booster failure, vacuum leaks and extra pedal pressure is that we are talking about a 1970's vehicle...

Having owned two of those older cars, and putting many miles on them, I learned that not only are there no sensors for wear, the calipers limit the travel of the pads so that the bare metal of the worn out pad can't touch the rotor. Remember he said the pads were worn pretty thin, but the rotors were still smooth? The normal symptom of exhausted pads is added pedal pressure and eventually some extra travel in the pedal. The pedal will NOT get spongy or soft, or have a lot of excess travel.

Once the pads are worn down on these older models, they limit in the calipers and the only braking action you get is by using excessive force to "bow" the pad out enough to touch the rotor. This force "bends" the center of the pads out just enough to achieve some braking action. Releasing the brakes allows the pad to return to it's original shape. Contined use of the depleted pads will generate extra space between the pads and the rotors causing a small amount of extra travel. Continuing to use the brakes in this condition can cause a caliper to eventually fracture, or for a piston or pad to get stuck. (This explains the stuck piston mentioned above.)

One other recomendation for the older cars... LOOK at your rear rotors once in a while. Those caliper pistons tend to stick and you don't really notice that the rear brakes aren't working until the front ones start acting as above... Just take a simple periodic peek at the rear rotors to be sure they aren't rusting over where the pads would normally be keeping them polished! I was shocked when I saw mine years ago and then I looked at my other car and it was the same way! Two cars, no rear brakes on EITHER CAR!

At least all of this was true for my 74 240D and 75 300D. I don't know about the other models. When MB started using the wear sensors I never let my pads wear down till they wouldn't work any more. My 83 had the sensors as well as everything I've owned since...

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with worn pads revisited:
my sdl has worn pads. the fronts are pretty gone and the rears are about 50 to 60% gone. i am going to replace all of them (with ceramic) and measure my pedal travel. btw at present my pedal feels "long". i will measure it before and after. to see if there is any reality to my feelings. then if it is true i will still wonder... how?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Pedal travel has nothing to do with whats left of the pads. The fluid level in the resivor drops as the pads wear thats about it.

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