There's a bellows/diaphragm in the cap that seperates the fluid from the air. The diaphragm extends as the fluid level drops. The only exposure to air is when nosy people open the cap to peek at the fluid instead of looking at the level through the translucent reservoir.

 

Now I had to get nosy and open the cap to see if there is a diaphram. You are right, sir, there is. Time to change the fluid

Given that, I think the only way to know for sure is to someone to send a sample of two year old brake fluid from the system to a lab for water content analysis.

 

Good idea, but there would also need to be a baseline sample to know what the water content was when new. I bet it's not zero. The delta contamination as a % and what this equates to in reduced boiling point would be interesting to see.

What's that old joke about a guy sending in a 'mixed' urine sample with a little of everything in it hoping to fool the lab and they came back saying "Your tap water has lead. Get a filter. Your dog has worms. Give him vitamins. Your daughter is on drugs. Get her in rehab. Your wife is pregnant. It's not your baby. Get a lawyer. And if you don't stop j*rking off, your tennis elbow will never get better."

 

And to be really pedantic, a new fluid that has been poured out of the container, it might pick something on the process.

 

Thinking about it, I think the first sample would have to be taken from the system shortly after changing the brake fluid. I am prettty sure there is some old fluid lurking around and that should be allowed to mix first (braking hard enough to let ABS kick in etc.). Then the second sample after two years using similar procedures. That should give proper delta %.

I don't know what the procedures should be to mix everything to get representative sample up to the reservoir. I would think that most of the time you just push the liquid little bit forward and backward.

 

Are you saying there's something in the cap that can extend by the whole amount the fluid moves from new pads+rotors to worn ones, and is air-tight? I don't think so.

Regardless, the fluid nearest/in the caliper does get contaminated or if it doesn't it sure behaves like it (see Bob's comments and peek at the colour when you bleed old fluid out).

 

We have the adjustable brake pedal on our '03 S Type 3.0 and noticed that the pedal is somewhat 'loose' feeling for the first part of the stroke. Other S Types that we test drove with that option did the same. That said, I did have the occasion twice in the last month to have to panic stop from a slow speed in good weather and the pedal went hard for a second or so then the brakes came on full. I've checked the booster and it seems to be working OK when I do the engine off pedal test. Once I start it up the pedal goes from hard to full on with normal pressure. This has happened only these two times and the result of a fast and hard pedal application. In both cases, I literally slammed on the pedal. That short delay seems like a year when you have something closing up on you.

Anyway, it appears the thread went in the direction of replacing fluid which was done within the last year in our case. The fluid is not low and otherwise the brakes are even and steady and in good shape.

It looked like you might have a couple of things going on. Has anything that you did up to this point solved the delayed response?

Regards,

Mike

 

When you braked so hard didn't the ABS operate?

 

Somehow or other moisture gets into the fluid (through the cap?) and the fluid really likes to absorb it. (Hygroscopic) Don't surpose it much matters how. Dot 3 & 4 absorb water and eventually the boiling point drops, dot 5 which is a silicone base does not absorb water "but", if any water does get to dot 5 it will remain as an unabsorbed liquid "globule" in the hydrolic system and can boil at a "very low" temperature which could possibly cause a catastrofic brake failure. For this reason it's recommended that we use Dot 5 in collector cars or vehicles that are stored for long periods, etc., Dot 3 or 4 in street cars and usually only Dot 4 with a high dry boiling point in a race or track car.
BTW, if anyone has ever changed the rear calipers on these cars, the air is really difficult to evacuate when charging them with fluid. JTIS explains the rather arduous procedure and failure to follow the directions to get all of the air out can cause symptems similar to what the poster was experiencing.

 

Dot 5.1 is not silicone based and is compatible with Dot 3&4 but with higher dry boiling point. I used that the last time I changed the brake fluid.

 

To help with the issue on my 3.0L Sport, I pulled off the front rotors and sanded them with garnet paper. This rid them of the really heavy pad glazing that was causing the brake shudder. It also appears to have helped a bit with the dead spot in the first 1-2" of travel.

In the spring, I plan to replace all the rotors and pads on the car, as well as bleed the system. It's too cold in MI now to do this in my unheated garage.

 

I guess my answer would be how do I know? These were stops from about 20 mph. When the ABS is 'working' does the pedal go hard? The behavior was that the car slowed down and stopped without locking the brakes except for at the very end of the stopping distance. The reason I'm inquiring is because the pedal went hard and it seemed like I lost braking power. Is that normal with ABS?

 

Typical ABS engagement would sound and feel like a buzzing or rapid tapping sensation from the front end.

The 'change the fluid' discussion above is essentially a red herring as related to the issue you've mentioned. Water is no more compressible than brake fluid or any other liquid. The fluid inside the braking system rarely reaches the boiling point, and then only inside the calipers themselves.

 

ABS reduces stopping distance (generally speaking) and feels as described above. If you've no idea what it feels like find somewhere safe to play that will be tough to brake on (ice, gravel, wet leaves, ...) and brake HARD. Repeat except HARDER until you feel the ABS. You'll not be in doubt when it does operate.

It's wise to test it every now and then - at least annually - because when you need it you want it to save you (could be your life).

 

Sorry to disagree with you "tot" but as far as I have ever known, DOT5 is a silicone fluid and definitly NOT compatible with Dot 3/4. I've been wrong before in my life, as my wife likes to occassionally point out, so please don't be offended if I ask where you have gotten your info. and I'll check it out. We only use Dot 4 (+-600deg.) in the track car, and I don't know of any Dot 5 fluid with a dry boiling point anywhere near that temp.

 

OOPS, Sorry "tot". I guess we're both sort of right. I just looked it up and Dot"5.1" (fairly new) is compatible with 3/4 because it is a glycol based fluid. However Dot"5" is silicone based and very uncompatible with 3/4. Mix even a small quantity of those together and you'll end up with a system full of SLUDGE!
I still don't know of any fluids other than Dot 4 that have dry boiling points up around 600 degrees which we use for racing. Maybe something will be coming out in the future.

 

I would not bother with sending any brake fluid samples around. All cars really need the brake fluid flushed every 2-3 years. It will get contaminated with water guaranteed.

DO NOT use DOT 5 in street cars. Beside the compatibility problem It has an additional problem of entraining air and you will fight a soft pedal forever. As has been posted it's only use is in collector cars that sit all the time and since it does not absorb water you don't corrode your brake system from the inside out.

The new DOT 5.1 is pretty cool and I have never used it yet. I use DOT 4 for the elevated WET boiling point. I view this as more important for a street car. The racers all focus on dry boiling points because they regularly change brake fluid thru the racing season. No has mentioned it but you can spend a LOT of money on brake fluid if you want. Check out Motul. Some guys use ATE and alternate between the blue and gold colors because it's super easy to see when the color changes while flushing.

Also be aware that unless the brake fluid is in a steel container it will pick up water sitting on the shelf in a plastic bottle. This is why some guys still swear by the factory Ford brake fluid because it still comes in a steel can. So don’t buy a bunch of brake fluid and store it away. Fresh is better!
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Again, I'd like someone to explain to me HOW the water supposedly enters the system since it's sealed hydraulically or hermetically at each end.

W/R/T to DOT5- not that I'd ever bother going through the motions to make the swap but there's hundreds of thousands of vehicles in regular service that use DOT5 fluid, not just collector cars in storage. The US Military mandates it's use in many vehicles. It can be tricky to avoid air entrainment during filling as you mentioned, I know of several people who heat the fluid to almost boiling prior to installation to drives off any air. Lot of work for little gain.

Never heard the story about partially empty plastic containers allowing ingress of moisture before. Why would a full bottle not be a problem?

 

Mikey;
I have to agree with you but will say that it does happen to every car I have ever worked on so I really don't know why.

The US military uses it because of the lack of corrosion. They really don't know anything about brake performance. There are no factory cars that come with DOT 5 fluid.

Does any car come with DOT 5.1? I don't know but would think that would be the best?

You guys got me thinking maybe I should change to DOT 5.1 just to see what's different.

No the plastic is what lets in the water. Comes thru the walls of the container. I now see Motul comes in a metal can.
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As the pads & discs wear, the pistons move further out. The fluid level in the reservoir drops. There is not a vacuum in that reservoir.

So, air has entered. It's not sealed.

Where am I wrong?