Brake Fluid Moisture Absorption Rates

Not open for further replies.
Sep 24, 2002
I saw this come up somewhere else and have wondered about this myself. Most discussions on comparing brake fluids tend to center around boiling points. I'm wondering if there is any quantified data regarding the absorption rates of various brands of DOT3 or 4 fluids? Obviously what I'm interested in is determining which brands have lower obsorbing of moisture over time and would be candidates for extended bleed intervals. Or maybe there is no real difference in the same circumstances? For example is Castrol LMA really any better? thanks
Pour some water into the bottle of LMA and see if it absorbs it any slower than any other brake fluid. I like Castrol brake fluid. I also like Valvsynpower brake fluid. But, I don't care for the LMA or Synthetic marketing labels. I haven't found a brake fluid that doesn't need regular yearly bleeds to keep it clean. I also siphon/refill the master cylinder and topoff at every oil change interval. This keeps moisture level low and cleanliness high.
All your DOT 3 and 4 fluids are poly glycol ethers. The 3 and 4 differ mainly in molecular weight. Differences in brands may more relate to additives including anti oxidants than other differences. Since they are all similar chemically, I doubt there is much difference in water absorption. Big thing is to keep the system sealed. Most of the master cylinders now have the translucent receivers that you can monitor the level without taking the cap off. This greatly reduces the exposure to humid air. Every time you apply the brakes, a little air is drawn in, but not that much. The rest of the system is sealed and should be left that way. The European spec fluids are somewhat different, but I am not sure exactly how. I am certain, Castrol LMA and the need for changing brake fluid are over rated. Messing with the brake fluid and bleed screws when it isn't necessary is more likely to cause problems than prevent them. Calipers don't last forever. Even if you can delay the pistons rusting, corroding, or swelling by changing fluid, the rubber rings eventually wear away. You lose much of the fluid when you replace/rebuild the calipers. It simply is not worth it to change it otherwise. You should never put a caliper on without generously coating the bleed screw with a silicone grease such as Sylglide. The seal is at the bottom of the screw. Leaving the, open to the air, threads of an unprotected bleed screw wet with brake fluid, is a sure recipe for a repair beyond many people.
Interesting question. I tend to ignore the dry boiling point and use the wet boiling point. The idea is that the wet point is more typical of real world results after the fluid has been in your car for a while. Sometimes, a fluid with a lower dry point actually has a higher wet point. This leads to 3 questions: 1. For the wet boiling point, what does "wet" really mean? (what level of saturation or contamination) 2. How long does the fluid take to get to that level of saturation? (it may depend on the car, as different braking systems might be sealed better or worse) 3. Is this time interval longer or shorter for different fluids?
To answer where the "wet" BP comes from, it's based upon letting an SAE graded triethylene glycol monomethyl ether reach 3.7% w/v under controled humidity conditions. Once the control substance reaches this point, the sample fluid (kept in the same chamber) is called "wet" and is tested accordingly. 'Kule could probably shed more light on how the brake fluid grade TGME stacks up to conventional fluids, but it seems to be equivalent (or close enough) to a general DOT 3 grade. That help any? ferb!
Great reponses - I too use Castrol and Valvoline brands and use teh same tricks unDummy uses - gee I feel like a copy cat! I have a feeling this is not an easily answered question. For the purpose of comparison I think we should presume same vehicle across multiple brands of fluid. Or same test conditions (open beakers in humid environment). I suppose a test could be done lining up a number of fluids and first testing their initial (dry?) boiling points. Then exposting them to an extremely humid air environment for a period of time and then retesting their now presumably web boiling points and noting the deltas to see how much each dropped it's boiling points...? I've seen more tests, articles and opinions on motor oils than my brain can handle but when it comes to brake fluids - all I see is some mention of boiling pts here and there but personally I've never run across any good empirical data on comparing 'absorption rates' as a function of time and uniform moisture in the environment. thanks,
Even though boiling point indicates some of the properties of the fluid. I reckon that it's a better idea to get a vaporization vs temp. curve for all brake fluid. It's no good to have a sharp cut out as well as having something like 60% of the fluid got vapourize at a v. low temp.
I'm a little confused systema - can you expand on what you are suggesting about a vaporization curve? My experience when I've boiled the fluid (on a track) was that the brakes 'suddenly' got mushy - there was NO subtle warning - it just suddenly boiled. For 10 laps it can be consistent - then all of a sudden whamo - a sponge for the pedal w/ minimal slowing.
I guess we should have some sort of graph show relationship of temperature and amount of break fluid vapourized at that temperature. Liquid will vapourize even the temp. is much lower than its boiling point, but just the rate of vaporization is different. A curve / graph showing temp. vs vapourization rate will give us a better overall idea on the performance of the fluid. (Just like what you see with some brake pads, a temp. vs friction / stopping power curve) Since most fluid has vaporization at temp. below their boiling point. I guess it's important to know by what rate they will vapourize in various temp. A graph will tell us what temp range the fluid work best and when their stopping power is going to diminish.
A) Is this true of a glycol fluid in a closed system? B) If brake performance does NOT degrade UNTIL the fluid actually boils in the caliper or slave cylinder - why do this? I'm curious what value is this data in a practical application of hydraulic activated brakes? thanks,
The problem is that no brake system is truly closed. Brake fluid's anti-corrosion properties and compressibility degrade as it absorbs water. There is a noticeable pedal feel difference between new fluid and 2 year old fluid. Don't know if you have ABS, but the actuator on my car lists at $1700 and is available with a discount for $1200. I don't play around when it come to less than $10 worth of brake fluid. [ December 03, 2003, 09:46 PM: Message edited by: VaderSS ]
When a brake system is at rest, it is open. When pedal pressure is applied, a brake system is closed. (except for the nonfunctional excess left in the reservoir) But I need some clarification on a few points you raised... Even if we only consider an open state with teh brake pedal at rest - it is only open at the master cyl reservoir. So as a function of temperature we move towards the boiling point - are you saying that vaporization occurs at the master cyl when the fluid in say the front right caliper boils? Have you ever boiled the fluid in your brakes? If so did you notice a gradual or analogous degradation of braking prior to boiling as it approached boiling? Or was the drop in performance acute? And are you saying that since brake pedal feel (performance?) degrades as water is absorbed - are you saying that water is compressible or that it's presence causes glycol itself to become compressible? thanks,
Brake fluid is hygroscopic. That means that it is almost akin to a vacuum when it comes to moisture. It will pull moisture past seals, and through rubber and plastic. When I think truly sealed, I'm thinking along the lines of a regrigerator cooling system. That is sealed. I've never boiled fluid. I have overheated brakes to the point where they stop working, but it was a gradual thing. When you boil the fluid though, you lose your pedal. I'm not sure of the mechanics of it though. I'm not sure of the mechanics of the compressibility degradation. I just know that it happens. The primary thing I am looking at, is the loss of corrosion protection. I've not had to replace any parts for brake fluid related causes, such as seal failure or internal corrosion of calipers or wheel cylinders or had to replace a master cylinder, since I started doing an annual brake system flush. That's good enough for me...
Heat degrades brake performance 2 different ways. Most common is over heating the lining enough for liquid to ooze out to the surface lubricating the lining. Given a chance to cool, it may form a glaze reducing braking even when cool. Sometimes the glaze can be sanded off, but the brakes may not really work right until the abused lining is replaced. Your metallic and semimetalics are more resistant to this. Extremely hard braking may heat things up to where enough heat is transferred to the fluid to raise the vapor pressure above atmospheric pressure. When you release the pedal and the pressure drops, vapor forms displacing the fluid back to the master cylinder. Next time you apply the brakes, the vapor compresses, maybe even reliquefying. However you run out of pedal before forcing enough fluid into the line to build any pressure. Water, brake fluid, and any combination of them are liquids, and never compress. The more water in the fluid, the lower temperature it takes to produce the compressible, brake destroying vapor. Since it is very difficult it keep all the water out of a real world system, brake fluids are designed to maintain a high boiling point even when contaminated by some water. Any air in the system causes the same problems. Other components flex enough to create somewhat of a soft pedal. Lines stretch, the flexible rubber ones the most, calipers and brake drums flex. The firewall gives a little. I worked in a brake shop specializing in older trucks for 3 years. The old coot that owned it had been there over 40 years. I never heard anybody mention changing brake fluid. I did hear a lot about bleeder screws with a broken Easy Out in them. He also strongly believed in rebuilding calipers and wheel cylinders every lining change. Rubber against steel wears away. It may even lose its elasticity eventually. If you want to play mechanic and change the brake fluid, go ahead. Just don't try to tell anybody it is necessary or likely to solve more problems than it causes. Leave well enough alone, especially if you don't know another way to handle a stuck bleed screw than pulling harder. I have never changed the brake fluid in my 77 truck or the 3 cars I traded off at 10 years or more, and I have never had to replace any corroded hydrolic brake parts. I do rebuild the cylinders and calipers now and then. [ December 04, 2003, 10:28 AM: Message edited by: labman ]
Hmmm, fluids have no compresibility...
 Compressibility of Brake Fluid at 1200 psi   
Temperature  Silicone Fluid   Glycol Fluid 
25°C/77 °F 0.85% ~0.3% 
66°C/150°F 1.00% ~0.3% 
93°C/200°F 1.23% ~0.3% 
121°C/250°F 1.53% ~0.3% 
149°C/300°F 1.84% ~0.4% 
177°C/350°F 2.11% ~0.4% 
204°C/400°F 2.41% ~0.5%* 
232°C/450°F 2.68% ~0.6%* 
260°C/500°F 2.89% ~0.7%* 
316°C/600°F 3.60% I don't have a similar chart for water, but it is compressible too;  - I'd guess that water is actually less compressible than brake fluid, but what does water do to the compressibility of the fluid itself. I think that it degrades it. I have seen seriously corroded(internally) calipers and wheel cylinders. When I've bled fluid that has been in a system for a decade or more, it's usually dark brown and very murky, usually opaque. Please don't try to tell me that that fluid should be left in, that it does more harm than good to remove it and keep the fluid fresh. Even 1 year old fluid is noticably darker than fresh fluid. It has undergone a change. How long CAN you leave the fluid in? I don't know, but an annual exchange sounds good to me, and I've never had it cause any problems and, in my opinion, has prevented problems.
Nothing is absolute, but the 0.7% max is negligible in the real world. The flexing of the caliper is a much bigger factor. Cylinders and pistons do eventually corrode. Usually in a system that has been let go until it started to leak. With occasional rebuilds, the fluid is slowly replaced eventually. Old boots let in more air. Maybe you haven't yet, but many people do get them selves in trouble messing around with stuff better left alone until you have to.
As I mentioned before, there is a noticeable difference in pedal feel between old and new fluid. I honestly don't care why or how it's there, but it is. Fresh fluid cures it. Most of the people who visit this site are not of the type to "leave well enough alone." We are here becuase we want the best for our cars. Leaving old fluid in is not best.
If you want to play mechanic and change the brake fluid, go ahead. Just don't try to tell anybody it is necessary or likely to solve more problems than it causes. Leave well enough alone, especially if you don't know another way to handle a stuck bleed screw than pulling harder.
According to my BMW owners manual yearly brake fluid flushes are part of the maintenance schedule. Hasn't caused me any problems. Don't know if it solved any, either. [Cheers!]
Not open for further replies.