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Old 08-07-2008, 07:05 PM
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Unhappy Brake Fluid and HEAT

Thinking about the recent thread regarding "apparent" brake fade on a hill climb, it occurred to me that the loss of braking effort and the increased pedal travel could have been caused by heat. Brake lines that are too close
to exhaust piping or headers can and do get hot enough to boil the fluid. It is possible that when mods were made to a hill-climbing Jeep that not enough attention was given to the location or RElocation of the brake lines. It just isn't likely that hill climbing would produce the kinds of brake drum temperatures that would cause the sysmptoms as described in the recent thread, so I'm looking elsewhere for the answer. I think heat is the culprit but it is from an external source, not from braking.
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Old 08-07-2008, 07:19 PM
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But, if that were the case, why only when on the hill? That's a whole lotta heat in a small amount of time to cause the problem.
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Old 08-07-2008, 07:29 PM
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It's possible, but it seems very unlikely to me. If you were to wrap the brake line several times around the pipe in front of the muffler, and then run the engine at full power for a while, yeah. Short of that I can't see it happening.

If the fluid is contaminated with water the boiling point is lowered, but even if it were pure water it would only go to 212, and I don't remember ever finding a part other than the exhaust system itself that I couldn't hold my hand on.

When the last axle bearing was grinding very obviously I checked the temperature of the housing at 175 or there abouts. I don't think that could generate enough heat to overheat the slave cylinder.

His experience does sound like boiling brake juice, but I just can't see a mechanism that could cause it under those circumstances.

And my experience is quite similar, but I can double-darn guarantee that there was nothing overheated in that case - I had just been puttering through a shallow creek in 1st low at idle speed for the past twenty minutes or so, and hadn't pushed the old girl hard for twenty years.
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Old 08-07-2008, 07:59 PM
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I doesn't take much moisture in the system to get some vapor. Any steel line that is close to an exhaust pipe can get past 212. It doesn't necessarily have to touch the piping, and could already be heated up before the Jeep even took a run at the cliff. It wouldn't show up unless the brake system was reeeeely needed. Anyway....it's just another idea. I used to be kind of nonchalant about how important it was to have moisture-free brake fluid. I got over that notion real quick when I got a truck brake system good and hot.
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Old 08-07-2008, 09:03 PM
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I would think that air moving over the brake lines as the Jeep moves (either forward or backwards) would cool the lines and the fluid somewhat.

To help with that heat transfer, you could install some finned tubing to increase the surface area.
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Old 08-07-2008, 10:05 PM
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The heat generated by exhaust is NOTHING compared to the heat that the fluid sees at the front brake caliper....

One of the key parameters of brake fluid is it's boing temperature.

This web site has a very good discussion of brake fluid and boiling temp.

As we all know, brake fluid tends to soak up the moisture in the air...hence, as it gets older (soaking up moisture), it's boiling temp.

As to boiling causing brake fade....NOT....think about it...if the fluid starts to boil, things would expand and it would start to push on the brakes when you don't want it to.....
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Old 08-07-2008, 10:18 PM
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A word of caution, be careful what you post. The natives are restless.

So you don’t get trounced on, I’ll tell you. The brake pedal is not depressed so there is a path open for the expanding fluid to the master cylinder reservoir.
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Old 08-08-2008, 04:07 AM
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Two guys were talking -
A asks: What's the smartest thing you've ever heard of?
B replies: A computer?
A: No, a Thermos! Ya put cold in it, it stays cold, put hot in it it stays hot.
How does it know?


How does it know only to make enough heat to fade when backing? And not after a prolonged dragging - they didn't work right when first applied when backing down.
Where did that heat come from? Who drags their brakes going up a hill?

If it was because the lines were too close to the exhaust, then why doesn't it happen in forward direction too - right after he made the hill, and went over the top?

Sounds like some people have never played on hills. Some here may never have even seen hills!

Solve the "how does it know" and you'll have the answer.

Funny how "I don't want it to be that" is so popular.
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Old 08-08-2008, 06:35 AM
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Remember RR, that the problem only showed up when the braking system was needed to hold the vehicle IN REVERSE. In that instance, all of the deficiencies came together; lack of self-energization; maybe less than perfect shoes and drums, AND... any fluid anomaly like moisture accumulation. In a forward direction in the Macy's parking lot it would not have been even noticeable. One of the few times I have had real scares due to lack of braking effort was when I was trying to hold a vehicle from rolling backwards on a steep slope. It definitely isn't a myth; FOR WHATEVER REASON, braking effort IS reduced under those conditions. The only brakes I have ever used that seemed to hold equally in either direction were S-cam air brakes.
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Old 08-08-2008, 08:14 AM
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Braking effectiveness is reduced going backwards, no question about that.

I think the main reason is contact - when the entire shoe assembly rotates in the reverse rotation, the shoes don't totally contact the drums. Not just on this one, but ALL Bendix drum brake systems.

Since we no longer use a shoe arc grinder to get the shoe to the same arc as the drum, putting on new shoes they only contact partially. They have to be "broken in" or "worn in" till they contact better. That's why the first 100 miles or so is important not to overheat them. But that's in the forward direction.
I'm sure everyone who has done brake jobs has seen linings that were not worn on the entire surface - the arc was not matched.
Replacement shoes are usually factory arced to about .040 oversize, they assume the drums have been turned. (Ever put on "replacement shoes" with a new drum? The shoes barely fit with the star all the way down.) In the "old days" we arced the shoes to the exact size of the drum - they worked so much better then. (Thanks Tree Huggers, we don't need brakes that stop anyway - please step in front of me.)

Even if the forward direction had worn in properly, when the shoe assembly rotates in reverse contact is from a different angle. The linings were "broken in" to the forward position, but now that the assembly is in a different position, contact area is greatly reduced. The shoes still contact, but not along the entire surface, just in one or two spots. That greatly reduces braking effectiveness.
Yes, since the contact area is smaller, it will build up heat faster - if it's used for a long time.

But - in this case - he applies the brakes AFTER storming the hill. While he was storming up, brakes were not applied, no heat there. The only heat present would be from the last hill climb - if he even used the brakes much it had time too cool.

The other possibility of heat would be from the exhaust - which of course is very hot from the "storming" up the hill. If the brake lines had been moved too close to the exhaust, brake "fade?" would be experienced in both directions, not just reverse.
Same way for water in the fluid. (How does it "know?")

If he's hill climbing in a competitive event - he'd have a fairly long wait between runs, letting the brakes get very cool.
Or if he's just playing on hills, like I do, he's using the brakes more frequently but not near enough to experience heat fade, no matter bad the brakes are.

And - he mentioned the pedal sinks at the same time - heat fade, at least with the heat fade I've experienced, the pedal did not go down, it stayed up, but the brakes just didn't stop like they should. Pumping or pulsing the pedal didn't help - it was overheated linings that caused it.

But - improperly adjusted brakes, including the parking brake, will do it every time!

Try it - loosen your brakes with the star wheel, after about 15 clicks each wheel, step on the pedal - first time goes low, subsequent pumps get it back up.
Drive it - notice if you apply the parking brake a little you feel the pedal get higher for a moment (you are biasing it off the normal "rest position.")
Keep giving it a little more parking brake until it drags all the time, then back it off slightly. Now you have duplicated Patrick's.

Now - back up and hit the brakes - make sure you have nothing behind you, NO BRAKES! Pump them a few times, they work again.
No heat involved here!

BTW - that's a diagnostic trick when diagnosing brake problems - without ever pulling a drum, you can tell alot with how the parking brake reacts.

But of course, - for those that "Don't want it to be that," it isn't.
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