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A dealer service mechanic told a friend of mine that the reason his rear break pads were worn out was due to old break fluid. Does anyone else think that this sounds odd? Maybe he just used the rear break a lot? :confused:
 

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Old brake fluid will have a lower boiling point than the fresh stuff, and maybe contain some water. It cannot possibly have any effect of the life of the brake pads, other than to possibly prolong it by preventing strong brake application due to boiled brake fluid.

I'd inform the dealer that they hired a monkey who has no idea of what he's talking about.
I'd also not trusty my bike to someone who clearly has no knowledge of something as simple as hydraulic brakes.

Oh yeah, also tell your friend he's been taken for a ride.
 

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I concur with the last appender.

Old brake fluid may promote corrosion INSIDE the brake-system but would have very little effect on speed of pad-wear.

When the pedal (or lever) is released, a small opening is revealed within the master-cylinder which allows ALL pressure in the system to bleed back to the reservoir.

In other words, you could have ANY fluid in the brake-system and it will not promote pad-wear because of the design of the system. When the pedal (or lever) is released, there is no pressure to make the pads drag.

With that said, if you do not keep the sliding-surfaces for the calipers/pads clean and free, the pads will tend to "stick" when you release the pedal (or lever) and make the pads drag. It does not hurt to perform a PM (Preventive maintenance) on the calipers/pads at least once a season to ensure the pads fully release.

Certainly when you are spinning the rear wheel to lube the chain, if you sense ANY dragging of the brakes, get it fixed.
 

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That advice coming from a mechanic is so strange that I'll bet your friend misunderstood. Maybe what the mechanic was saying was that if the rear pads are that worn the fluid is in need of change as well -- which is a reasonable comment.
 

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Our standard polyglycol ether brake fluid is hygroscopic--it absorbs moisture from the air. When we brake very hard the whole brake system heats including the fluid with heat transferred from the pads through the pistons to the fluid. If the fluid contains moisture it'll flash to vapor at a lower temperature than when new, your lever or pedal goes all the way down, and you have no braking. Crash. DOT4 fluid has a higher wet boiling point (3.7% water) than DOT3...minimum of 155°C vs. 140°C for DOT3. Better brands of fluid have even higher boiling points.

The minimum wet boiling point for DOT5.1 fluid is 190°C. DOT5 silicone fluid has a spec of 180°C, and must not be used in systems designed for ordinary brake fluid like our bikes and cars. Use DOT5 fluid only in systems designed for it.

By the way, all brake fluid is synthetic. Don't be hyped by a label.

That mechanic was either stupid or crooked.
 

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I have seen where the small hole in the master cyl. is plugged because of old fluid. This does not release the pressure BruceP spoke of and the brakes drag. The calipers can also stick because of old brake fluid. Not all dealers and mechanics are crooked. He may have been trying to prevent the new pads from wearing out prematurely.
 

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I have seen a calliper cylinder that wouldn't release properly due to corrosion from water in very old fluid (around 15 yrs), so it could wear the pads if they drag continually - but a frozen brake is pretty obvious and you'd be doing something about it before the pads wore more than a tiny amount. At a guess I'd say a lot of bikes never have the fluid changed from sale to wrecker's yard and internal corrosion (rather than external from road salt etc) isn't all that common.
 

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No expert but isn't the break system a closed (sealed) environment? It seems to me if old break fluid heats up more than new, can't the higher expansion of the fluid cause the breaks to be partially engaged?
 

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Old brake fluid doesn't heat up more. It is just more likely to vaporize at the same high heat where good fluid remains liquid and continues to do its job.

Yes, corrosion in brake parts certainly can be a problem, but changing the fluid doesn't make the corrosion nor the trouble it causes go away. Do renew the fluid periodically to avoid these problems.
 

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No expert but isn't the break system a closed (sealed) environment? It seems to me if old break fluid heats up more than new, can't the higher expansion of the fluid cause the breaks to be partially engaged?

Closed (sealed) environment? - YES - it is as "closed" as humanly possible... but moisture from the air migrates thru the smallest of openings. Brake fluid is hygroscopic (by design) so it will "sponge up" any moisture which finds its way into the system.

Partially engaged? - NEVER - please reread my last post explaining about the little hole which is ALWAYS there when the pedal/lever is not in use. This is intended to "bleed off" any residual pressure as soon as it tries to build.

BTW: The hygroscopic nature will protect your brake-system from corrosion until it is saturated... at which point, liquid water droplets will form in the lowest part of the system. (the calipers) and corrosion will start.

This is the reason that brake fluid MUST be changed every 2-3 years. (never based on miles!) After several humid summers, your brake fluid gets saturated with water.

Dont let anyone talk you into using "DOT5" brake fluid. It is mostly silicone oil which is NOT hygroscopic. Any moisture which finds its way into the system IMMEDIATELY forms water-droplets and corrosion starts. DOT5 is intended for racing where the fluid will be changed out after every race.

What else do you want to know about hydrolic brake systems? :beatnik:
 

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A few systems are designed for DOT5 brake fluid for everyday use, Harley-Davidsons for example, but I guess that is just every sunny, dry day.

DOT5 silicone fluid is more compressible than ordinary brake fluid. (All fluids are compressible, but not much, so we say that they aren't.) DOT5 fluid will feel spongy, and it won't work right in ABS systems designed for ordinary fluid. DOT5.1 fluid is ordinary polyglycol ether base, but very high boiling point--good stuff, not silicone.

To achieve the higher boiling points in DOT4, DOT4+, and DOT5.1 fluids, other chemicals are added, mainly some borate compounds, that are sometimes incompatible with the elastomers in systems designed for DOT3 fluid--most cars.
 
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