Disc brakes 101

[jsharp]

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Originally posted by labman:

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Originally posted by JDP:
Based on the logic used here against flushing and changing brake fluid, the only time a change of antifreeze is needed is when a rad or heater core is replaced due to corosion/pin holes or the water pump seal leaks. Change the engine, transmission and axle oils only at rebuild time and UOA is a waste of money in trying to prevent the "unlikely" snip...

JD

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Antifreeze, engine oil, etc. actually contact the parts that most commonly fail in those systems. Most brake problems come from poorly maintained hardware or corroded pistons. Most of the corrosion of the pistons comes in the narrow band near the seal, mostly outside the seal, or at least deep in a narrow annular space likely not touched by flushes. You flushers' uncritical acceptance and fanatical insistence of it obscure real problems and reduce credibility about other real world problems. If ''experts'' insist on replacing brake fluid when not doing it seldom creates problems, why trust anything else they say?

There is a large body of data showing problems with not changing antifreeze and engine oil in a timely manner. The reports of problems from old brake fluid are so few compared to the millions of cars with old brake fluid as to suggest that in many cases, something else may be involved. Many of the arguments I read here suggest wide ignorance of brake systems.

Wet DOT 3 brake fluid has a boiling point of 287 degrees F. That isn't far below the heat distortion temperature of aluminum or what it would take to degrade other components. I am sure the pistons get hotter than the brake fluid as they transfer the heat to the fluid. We need to differentiate between problems and bogeymen. No Chicken Little, the sky won't fall if you don't change your brake fluid.

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I really doubt if you'll ever see boiling problems from the lowered boiling point of brake fluid that has absorbed some water in a street car. The components just don't get hot enough. A car that sees track use is a different case for sure.

I'd be more worried about possible internal corrosion from the old fluid. In 30+ years of working on vehicles though I've yet to see it be an issue...

 

[ChrisW]

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Originally posted by jsharp:

I'd be more worried about possible internal corrosion from the old fluid. In 30+ years of working on vehicles though I've yet to see it be an issue...

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I flushed the brake fluid from my mother's 97 Honda Accord EX a couple months back, and the old fluid was dark, somwhat rusty colored and had [metallic] particles floating around in it. I am almost sure [moisture] contamination had caused some sort of corrosion in the system.

The brake fluid was never changed prior (we bought the car used); having known that, I could have possibly prevented what now may need to be a new brake master cylinder, and a possible rebuild of the calipers. Dust seals and what not are still good; I inspected those visually when I replaced the pads at the time of the fluid change.

BTW I used Valvoline Synpower brake fluid, good stuff imo.

 

[kreigle]

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Originally posted by jsharp:

I'd be more worried about possible internal corrosion from the old fluid. In 30+ years of working on vehicles though I've yet to see it be an issue...

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I have.

I have had to throw away calipers and wheel cylinders because the part of the piston behind the seal (the part that is bathed constantly in brake fluid) had corroded to the point that it stuck in the cylinder. This was not a case of bad seals, as they did not leak fluid at all. they froze up solid and would not stop the car. This was in when I had a 1969 Saab (1981 - 1990)-every 3 yrs the wheel cylinders would sieze up. I was too stupid then to change the fluid more often than that.

 

[kreigle]

1. Disc Brake Quiet - the "glue" that is sometimes recommended (sometimes not) on pad backs. Should this stuff be used on ALL applications, or do just some manufacturers recommend it?

I have personally never used, or needed this stuff, YMMV.

2. Caliper pin grease (where the caliper pins slide within sealed boots). Do you remove, clean, and relube every brake job? What kind of lube?

I've been checking the pins every time I change the brake fluid. Whe I got my car, the pins were rusted into the brackets and were a pain to remove and clean.

3. Pad contact points/pins, etc.(where metal pad backing plate slides against caliper) - some people/brands recommend antiseize on these "exposed to the elements" areas. Is this a common or recommended proceedure for every application?

I never did until I had the dealer replace my pads 4yrs ago. Unfortunately, they put too much on and ruined the pads, the contact surfaces were coated with grease.

Since then, I've cleaned and 'lightly' anti-siezed the sliding surfaces twice - once when I put the replacement pads on, and again this year when I replace my 2yr old *black* brake fluid.

 

[kenw]

I've never had a problem with "old" fluid, and only bleed on occassion. Meaning I try to with every brake job, but won't guarnatee that I have 100%, either. But then, i've never owned a Saab, either.

The Disk Brake Quiet is a RTV-like substance that is used if the car does not already have the thin metal "shims" already or if the replacement pads don't have them. I've skipped using it at times, and had squealing brakes, and then others times not. Use it and be sure, it's cheap.

I usually check the "ease of movement" of the caliper pin with each brake job, if it feels tight or jerky, or if the grease looks bad (whatever that means at the time!), I'll pull it, clean the hole and pin and relube it with bearing packing grease. It takes all of about 30 seconds.

I've never used anything on the sliding areas. And I've never seen anyone use anything there either. Anti-seize will disappear in the first puddle.

 

[Kestas]

Antisieze will not disappear with the first puddle. I serviced the brakes on my Mercedes and found that the antisqueal paste used behind the pads (by design) is actually an antisieze compound. On top of that, the steering stops each have a dab of copper-based antisieze.... and that is a high-wash area! I know this because out of curiosity I reverse-engineered the material.

Don't use bearing packing grease for the caliper slides. That WILL wash out. Plus, I don't think it's good for the elastomer. Silicone grease should be used there. It's commonly sold as Dielectric grease in the auto stores.

Whatever you use behind the disc pads, use it!... squealing brakes are the number one reason for return visits to a shop after a brake job.

 

[jsharp]

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Originally posted by kreigle:

quote:

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Originally posted by jsharp:

I'd be more worried about possible internal corrosion from the old fluid. In 30+ years of working on vehicles though I've yet to see it be an issue...

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I have.

I have had to throw away calipers and wheel cylinders because the part of the piston behind the seal (the part that is bathed constantly in brake fluid) had corroded to the point that it stuck in the cylinder. This was not a case of bad seals, as they did not leak fluid at all. they froze up solid and would not stop the car. This was in when I had a 1969 Saab (1981 - 1990)-every 3 yrs the wheel cylinders would sieze up. I was too stupid then to change the fluid more often than that.

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Maybe I've been lucky then. I've yet to need to replace any wetted brake parts that weren't at least 10 years old. I'm not sure changing the fluid out every couple of years would have made them last much longer...

 

[doitmyself]

Thanks for the responses. Very interesting.

Can someone explain what causes brake squeal and how do those thin shims stop it? Do the pads somehow shake or shimmy within their caliper mounting?

On my Escort brake job, I found the bottom caliper pins to be very stiff, in spite of ample lube within the one piece boot. Salt water had seeped in between the boot and caliper hole, rusting the hole and "sqeezing" the boot. After a thorough cleaning of the caliper hole, the pin moves like butter.

Are brake shops this thorough?

 

[Kestas]

The brake shops that are thorough will replace everything - pads, rotors, and calipers - and passs the cost on to the customer. They don't waste their time. Then there's no question of the brake job lasting a good while until the next service. The average customer doesn't want to even think about having their brakes serviced again until after 3 years or so.

Brake squeal comes from the fact that relative to the caliper there's always a slight bit of wobble in any rotor. That means that during braking the pads will vibrate sightly at the caliper/pad interface causing a squeal. The paste, shim, or compound is to take up this vibration, dampen the noise, and reduce or eliminate the squeal.

It won't do anything for a groan or squeal you may hear at creeping speeds that come from the pad/rotor interface.

 

[labman]

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Originally posted by Kestas:
The brake shops that are thorough will replace everything - pads, rotors, and calipers - and passs the cost on to the customer. They don't waste their time. Then there's no question of the brake job lasting a good while until the next service. snip...

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This protects the shop from an unknown history. It is hard to tell by looking the outside of a caliper what it is like inside. You can see the boot doesn't have any cuts or tears, but you can't tell if it is old and no longer has the elasticity to seal to the caliper. You can't see how much grit, salt, and moisture has already worked its way in. Quickest and safest thing to do is to drop on a couple of loaded rebuilt calipers, and a quick bleed. New hardware completes the no come back job.

Rubber that looks good may be shot. Many times I have pulled a leaking cylinder apart, and the cups looked good. Even when held up against a new one, they didn't show any wear. They had just lost their elasticity over time. A good honing, a little Sil-Glyde, new rubber parts, and a customer that didn't bring it back again.