Oil Life Question

[shorod]

I'll chime in again. While there is a seal on the master cylinder cap, the fluid can still absorb moisture. I'm a believer (now) in flushing the brake fluid regularly. Like most people, I had never changed the brake fluid in my Stealth twin turbo. I took the car to a local track to see which had more guts, me or my car. After a couple of laps to get familiar with the track, I started to push the car a bit. I got to the end of the straight, going around 130 mph, then stabbed the brakes to make the 90 degree turn, there was virtually nothing there brake wise. I mowed a little bit of the grass and putzed back to the pits.

The moisture in my brake fluid turned to steam which easily compresses, causing poor braking. After the car sat for a few hours, the brakes were back to normal and I was able to drive home. During the 4.5 hour drive I had no issues. Under normal driving, no issues. But, after that one experience, I now flush the brake systems in my cars as soon as the fluid starts to noticeably change colors. I also make sure to activate the ABS to get fresh fluid in the ABS pump so the moisture won't corrode the valves, etc. in the expensive ABS pump.

I've since had the Stealth back at the track and never experienced brake fade again.

BTW, the car had more guts than I do, especially with sticky tires installed.

-Rod

[MagicRat]

Originally Posted by shorod

I'll chime in again. While there is a seal on the master cylinder cap, the fluid can still absorb moisture. I'm a believer (now) in flushing the brake fluid regularly. Like most people, I had never changed the brake fluid in my Stealth twin turbo. I took the car to a local track to see which had more guts, me or my car. After a couple of laps to get familiar with the track, I started to push the car a bit. I got to the end of the straight, going around 130 mph, then stabbed the brakes to make the 90 degree turn, there was virtually nothing there brake wise. I mowed a little bit of the grass and putzed back to the pits.

The moisture in my brake fluid turned to steam which easily compresses, causing poor braking. After the car sat for a few hours, the brakes were back to normal and I was able to drive home. During the 4.5 hour drive I had no issues. Under normal driving, no issues. But, after that one experience, I now flush the brake systems in my cars as soon as the fluid starts to noticeably change colors. I also make sure to activate the ABS to get fresh fluid in the ABS pump so the moisture won't corrode the valves, etc. in the expensive ABS pump.

That's a good point. As you say, periodic changing brake fluid is necessary but is ignored by most people, (imo) partly because very few manufacturers place it in maintenance schedules.

Face it, there are many people who think that regular oil changes are a mechanics scam to make money. . Convincing these people that they should spend money to flush brake fluid when no manufacturer suggests it would never fly.

In the classic car world (aka old crocks) most owners change brake fluid regularly or put in DOT 5 fluid, which is silicone based and does not absorb moisture.
(But I read that it is more viscous than regular fluid and does not work as well in some systems)

[curtis73]

Agreed. Every other brake pad change I flush the brake fluid. Most of the "wet" fluid is in the master cylinder, so its away from where heat can cause boiling and loss of braking, but water in the master cylinder can cause rust where you don't want it.

The hydrophillic nature of glycol fluids was the main reason behind developing DOT 5 silicone brake fluid. The problem is, silicone fluid requires specific brake parts that won't get eaten by the fluid.

Glycol is cheap, and its not that tough to replace.

[KiwiBacon]

Brake fluids are hydrophillic by design.
If they weren't, then any moisture which got in the system would sit at the lowest piont (your brake calipers, which are also the hottest point) and would boil at around 100 degrees C.

But hydrophillic brake fluids can absorb some moisture and keep a boiling point well above the boiling point of water.
Brake fluids are rated with two boiling points. Dry and Wet.
Dry is devoid of moisture (like when you first open that brand new bottle), wet is fully saturated but still holding all the moisture in suspension.

As the others have mentioned, keeping your brake fluid fresh and dryest is the best way to keep the boiling point up so they don't boil and fade.

As for engine oil. If you really want to know, then pay to get your oil sampled at the recommended change interval. This will let you know exactly how the additives in the oil are holding up.
It'll cost a similar amount to an oil change, but it has the potential to save you many un-necessary oil changes.

FWIW, I never change my vehicles oil at 5000km (3,000miles). Two of them get oil changes at 10,000km, the other 20,000km.
The militarys that use the same engine as my 4wd change the oil at 16,000km (if the military doesn't count for severe use then who does?), I change it at 10,000km as it's easier to remember.
The shopping basket's recommended interval is somewhere around 14-17000km (can't remember, not looking it up right now), 10,000km is easier to remember and I'm making up for an unknown service history.
My work car is recommended 20,000km intervals.

[CL8]

Thanks guys for the replies.

I'll chime in again. While there is a seal on the master cylinder cap, the fluid can still absorb moisture

Now I was taught, through driver instructor education, that, that seal on the brake fluid cap was to keep DIRT out, not moisture out. I guess I was taught wrong, or is keeping dirt out a factor too?

Anyone have any suggestion on how often the brake fluid should be replaced?
(Maybe we need to start a separate thread about brake fluid.)

[shorod]

I wouldn't say you were taught wrong. I didn't specify what the seal was for, but implied it wasn't for moisture. That just happened by coincidence though and you kindly left me an "out."

The old rule of thumb used to be the brake system should be flushed and new fluid installed yearly. I doubt that frequency applies when DOT 5 fluid is used, and the materials used today may be sufficiently better to increase that interval.

-Rod

[jdmccright]

This discussion sort of morphed into a brake fluid convo, but from what I know, not all master cylinder caps are airtight. Some will allow air through a small valve or cap to equalize the air pressure as the fluid is drawn into the system from brake pad/shoe wear and operation. Because of this, fresh water-laden air does come into contact with the fluid regularly.

I've also had some MC caps that are truly airtight...as the fluid is drawn down, the rubber seal inside the lid has those nubs that extend down to fill the void. I recall these being better, though messier when you remove them.

[KiwiBacon]

Originally Posted by jdmccright

I've also had some MC caps that are truly airtight...as the fluid is drawn down, the rubber seal inside the lid has those nubs that extend down to fill the void. I recall these being better, though messier when you remove them.

Another example of an "airtight" system. The hayes brakes I use on my mountainbikes have a completely sealed bladder as the reservoir on the master cylinder.
As well as keeping the fluid away from the atmosphere, it means you can turn the bike upside down without worrying about fluid spills or air entering the system.

[CL8]

Originally Posted by KiwiBacon

Another example of an "airtight" system. The hayes brakes I use on my mountainbikes have a completely sealed bladder as the reservoir on the master cylinder.
As well as keeping the fluid away from the atmosphere, it means you can turn the bike upside down without worrying about fluid spills or air entering the system.

Are you an Evil knevil type that does tricks like that while riding your bike?

(Did he ever do flips in the air with his bike?)

[KiwiBacon]

Originally Posted by CL8

Are you an Evil knevil type that does tricks like that while riding your bike?

(Did he ever do flips in the air with his bike?)

Yes, but never intentionally.