Rear shock fluid

[CBUEB1771]

I have wondered: why do I care about water ingress in a closed and sealed system like this or the brake system.

The braking system is not sealed. The reservoir is vented to atmosphere to allow displacement of brake fluid. The atmosphere contains water vapor. Relative humidity and temperature change so water can condense on the inside of the reservoir. Over time enough water can get in to form a complete blockage under freezing conditions, unless the water remains in solution in the brake fluid. Sometimes people even drive through puddles. It is more of an issue with naval aircraft where cycling through high humidity and freezing temperatures can happen several times in a day. We certainly do have an odd language with adjectives such as hygroscopic, hydrophillic and hydrophobic. Browsing through Webster's it seems that hygro is applied to uses involving atmospheric humidity and hydro to more general uses involving water.

[bill308]

Thanks for the correction elj221c. You are correct, the term is hygroscopic.

I have never heard of a case where the brake fluid absorbed enough water to freeze into crystals but I believe it is possible. On the other hand, I have seen damage due to corrosion in calipers and wheel cylinders. You want the bore to remain smooth and you also don't want the seal to swell or deteriorate due to exposure to the damping fluid. This later item is worth checking into too.

Silicone based brake fluid (DOT 5?) does not have this particular short comming (not hygroscopic). I'd probably still go with the oil, at least there is some choice in viscosity so some possibility for adjustment of damping rate. Whether or not it causes the seal to swell or otherwise deteriorate, I don't know.

I guess if it was me, I'd go for the inserts and use a light oil to take care of the heat transfer from the shock insert to the strut tube.

Bill

[elj221c]

Oops, that will teach me to correct people!
"Do use propert damper fluid, not brake fluid."
Bill, I think the usual thing is to use antifreeze between insert and strut. I'm not sure if that would promote corrosion or not as I haven't removed my Konis since they were installed in '82.

Roy
'65S2

[elj221c]

Oops, that will teach me to correct people!
"Do use propert damper fluid, not brake fluid."
Bill, I think the usual thing is to use antifreeze between insert and strut. I'm not sure if that would promote corrosion or not as I haven't removed my Konis since they were installed in '82.

Roy
'65S2

[john.p.clegg]

Besides the effect of freezing doesn't the high temperatures reached by the brakes cause problems with water in the brakefluid?....I'm sure I've seen steam rising from my master cylinder a time or two.....

John

[CBUEB1771]

I have never heard of a case where the brake fluid absorbed enough water to freeze into crystals but I believe it is possible.

Are you willing to accept that possibly the chemists and thermodynamicists who have developed brake fluids over the years might have actually known what they were doing? That possibly brake systems don't freeze because because of the hygroscopic nature of glycol-based brake fluids?

[CBUEB1771]

Besides the effect of freezing doesn't the high temperatures reached by the brakes cause problems with water in the brakefluid?

The thermodynamics of mixtures is pretty complicated. In general, dilute solutions behave the way the dominant component does. With a small amount of water in solution in a large amount of hygrocopic brake fluid, the freezing point of the water will be depressed and the boiling point elevated. We get a benefit under both conditions. The reservoir caps on my 2004 Acura with the $Brembo$ calipers say "DOT 3 Or 4 Only", I guess the hygroscopic stuff is still considered good in some circles. OK, enough anorak stuff from me.

[bill308]

We're getting off topic here but water in the brake fluid is a very bad thing. Not only is corrosion a resulting problem but it significantly reduces the boiling point of the fluid. Dry and wet boiling points are routinely cited when talking about brake fluid for competition use and when the brakes get good and hot the pedal feels spongy at the onset and in extreme circumstances you will loose nearly all breaking force. Water in the fluid is changing to a vapor (gas bubbles) and unlike a liquid, a gas compressible. Racers routinely change break fluid just before a race for this reason. Some clubs make it a requirement to change brake fluid before allowing one on the track. Break fluid is never again as good as when it first comes out of a sealed container. It begins to absorb moisture immediately, but very slowly and it takes a while to work its way down to the calipers where the heat is absorbed. In normal street driving, especially in a light weight car like the Elan, this is not usually a problem because the brakes are really pretty good and they seldom get really hot, but if you push it, the problem will become apparent and usually at the worst possible time.

Bill

[reb53]

Bill,

You're absolutely right, this isn't hypothetical as I've had it happen.

Years ago, with what was probably the original fluid (!), out for a fast drive and the brakes disappeared.
Re-appeared a few white-faced miles down the road as I headed home to change the fluid !
Never happened again as I'm now not so casual about such maintenance.

Ralph.