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Old 02-25-2003, 03:03 AM
1stimer's Avatar
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Join Date: Oct 2002
Location: Pasadena, CA
Posts: 556
yid brakes

Although I am probably the most inexperienced wrencher on this site, I can tell other people where to look for information. I apologize to member who I stole this from.

I have read with great consternation all the discussion about bleeding brakes and sticking or dragging brakes over the last few days and weeks and began to question the methods I have used for three decades, without any problems. I still think it works fine and is technically sound so I will offer it here for your evaluation and use if you choose.

First, I jack the car up and take off the wheel to get good access to the caliper. Then I loosen the bleed screw, which I believe is a 9 mm hex. I use a box end wrench, which I slip past the hex, then put a tight fitting brake hose about 12 inches long on the bleed screw nipple. The other end of the hose goes into a pint or quart jar, preferrably something small in diameter (three or so inches) and around six inches tall. I open the bleed screw a few turns and look for brake fluid to run out so it covers the end of the hose. Then I fill the fluid resevoir and go back to the brake.

I take the mechanical stuff holding the brake pad in the caliper off (cotter pins, then the larger pins that go through the pad ears and into the other side of the caliper, and the spring mechanism to keep things tight and finally the electroes for the brake pad wear indicators). I then loosen the bleed screw again and use a big Craftsman screwdriver to pry the old pad back from the disc. I take care to avoid damaging the disc and don't care what I do to the old pad as I am throwing it away when I am done. I make sure I get the piston in the caliper all the way back into caliper. Then I close the bleed screw and remove the pad and start cleaning the openning.

Once the openning is clean, I trial fit the first quarter inch or so of the new pad into the openning. Sometimes it hangs up a little as the clearances are very small, and the finish on the caliper is pretty rough. Once I have confirmed the piston is all the way in and the openning is clear, I put the goo on the back of the pad and insert it all the way in the caliper.

Then I go to the other pad and repeat the process. I like to use a clear bottle to catch the old brake fluid as I get a good feeling from seeing the old black stuff (not really black, but it is far from the clear stuff in the new bottle) coming out knowing I will flush the line and have clean, fresh stuff in there when the job is done. Brake fluid (I like Castrol GT LM DOT 4 (LM is for low moisture absorbtion) fluid, and it is a couple of bucks for pint bottle) is cheap, and if it is discolored I think it should be changed. I have never seen a set of pads wear out without turning the fluid in the caliper from clear to something approaching black, so saving the old fluid is not a prioity. It also makes pushing the piston back into the caliper on each side much easier.

Yes, you have to pay attention so you don't get the reservoir too low and put more air in the system, but then you are working on a safety system on the car, and you should be paying attention. When both new pads are in I bleed any air from the system out the bleed screw on the caliper. If the hose in the jar is below the fluid level in the jar, the way this works is you fill the reservoir, open the bleed screw and pump the brake a few times. Then I close the bleed screw a little, and pump the brake a few more times, and then I close the bleed screw and pump a few times to see if the sponginess is gone. It is usually gone in one or two cycles of this. Now that my boys are in their early teens they get interested and come out to help. That speeds things up quite a bit as I can have one of them pump the pedal while I manipulate the bleed screw.

I repeat this process on each wheel, and usually consume two bottles of brake fluid for the whole job. The pistons go all the way back in the calipers and I do not have a problem with the piston to brake disc dimension when I put the new pad in the openning. I often do have some trouble getting the pad in, but as I mentioned that is the top to bottom dimension, and the overall clearances/surface finish. I will tap with a hammer when it gets tight once the pad is more than half way in, which ususally only lasts while the pad transits a "high spot" on the top or bottom lands of the caliper. I believe the tight clearances are necessary because the dimensional stability of the geometry of a new pad in the slot is reduced as the pad wears and it might get cocked in the openning, causing uneven wear or worse, damage to the disc or jamming of the works.

I have had one brake caliper fail on my old 1975 240D (the first car I bought, which I bought new after working in Alaska on the pipeline in the early '70s) when it had around 240,000 miles on it. It was the front left brake, and when I investigated I learned of the function of the piston to caliper seal in the releasing of pressure on the disc. I have read in the forum where people experience binding brakes and look to the hoses and master cylinder for solutions to the problem. I learned the hard way, but once I learned I find the issue very logical.

When you step on the pedal there is a vacuum assist to add to the force your foot puts on the piston in the master cylinder. The travel of the pedal under this pretty high pressure is what is needed to move enough hydraulic fluid from the master cylinder to the back of each pad (8 total) on each wheel, and push it into the disc with enough pounds per square inch force to quickly turn all the mechanical energy of the rotating wheel and car inertia into heat. The amount of fluid that actually moves is very low, and it accounts for the elasticity in the system as well as the pad material that is burned off. All in all the forces generated are huge, and when you take your foot off the brake, the pressure goes to atmospheric pressure (zero delta pressure within the system beyond static head of the reservoir height above the caliper) and once again there is next to no flow of fluid. To move the piston back in, as you have experienced trying to move the piston back in to change pads, takes considerable force. Which is not available from the hydraulic system design when you take your foot off the brake. For one thing, the reservoir fills any "missing" fluid, so there is no vacuum created to suck the pistons back. And it is doubtful a vacuum could do the job (it would be something like 14.7 psi max on the surface of the piston) and it is definitely undesireable that the piston retract significantly into the caliper as that would require a lot of hydraulic fluid to be pumped back into the caliper to activate the brakes. In addition, this is not how the system works as we all note when we change pads, the pad is always right up against the disc (few mils of clearance at most) and the pistons are extended from the caliper cylinder bores.

So, the only thing that pulls the piston back into the cylinder of the caliper is the little square cross section "O" -ring seal, which gets deformed during the braking event, and when you take the pressure off, it pulls the piston back far enough to stop the hard rubbing of the pad on the disc to slow the car down. When this seal gets worn, there is nothing to pull the piston back, so the pad stays in contact with the disc, generating heat that increases the contact pressure, and creates more heat, and so on.

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RT


1984 300D---Sold
1995 E420 - 106K - Brilliant Silver Metallic/Orion Gray --- Sold
1984 300SD - 151K Manganese Brown Metallic/Mostly Black --- Sold
1982 240D - 79K - Manila Beige/Palomino---Sold
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  #2  
Old 02-25-2003, 03:07 AM
1stimer's Avatar
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Join Date: Oct 2002
Location: Pasadena, CA
Posts: 556
Let me know if this isn't what you had in mind and I will delete it.

Let me know if you like it and I'll post some of the others I have saved.

__________________
RT


1984 300D---Sold
1995 E420 - 106K - Brilliant Silver Metallic/Orion Gray --- Sold
1984 300SD - 151K Manganese Brown Metallic/Mostly Black --- Sold
1982 240D - 79K - Manila Beige/Palomino---Sold
Reply With Quote
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