buns

For those who may wonder (as I did), how much moisture Dot 3 and 4 fluid can absorb in a short period of time, this test may answer that question. I filled a vial with Dot 3 and let it sit for two days. Pictures below- before and after.






.

GTOguy

When exposed to moisture, DOT 3 or 4 will absorb it. That's why it comes in sealed containers and brake master cylinders have air tight caps with diaphragms to displace air. Dot 5 is NOT allowed in modern ABS brake cars due to its inability to absorb moisture, which occurs in any brake system. As a result, the heavier water sinks to the bottom of the modulator valves, causing them to fail. DOT 3 0r 4 fluid hold the water in solution, so that no pure water pockets form in the system. Whatever the fluid used, it is best to change it out every 3 years or so, depending on location. Out west where I am in the center of the state, I get 10+ years out of it. When I was on the coast, nowhere near that.

63 340HP

Location, location, location... or better said environment, environment, eenvironment must be considered with regards to brake fluid contamination. The environment concern is due to the presence of water in liquid and vapor form. Proximity to water, and high water vapor pressure (humidity) effects the absorption of water into the brake fluid.

Water boils when hot, expanding 1600 times its liquid volume after a steam phase change. A little water in brake fluid can boil and expand, and lock up the brake system. There is more involved than atmospheric pressure cooking of hard boiled eggs, but the effects can ruin your day.

Water also invites corrosion. Corrosion takes years, and longer with modern stainless steels, but it is the silent hidden problem with storage.

One water problem ruins your day quickly, the other over years of the vehicle life.

Garage queens and those vehicles parked at the Armory and desert do not get out much. Corrosion from moisture during storage is the priority concern. DOT5 is preferable for this use to counter corrosion. High temperature performance is not the concern, it is a secondary benefit of the low moisture potential in a properly bled DOT5 system.

DOT5 silicone based fluid has the primary advantage of not being hydroscopic. Simply put, DOT5 does not absorb and retain water at a molecular level. Any water into a closed system with DOT5 separates out. Water is heavier than DOT5, so it settles out below DOT5 in claiper and wheel cylinder voids.

Good installation practice to bleed all air and water from the system is critical to success with any brake fluid, but more so with DOT5. Bleed air from the top of piston bores and bleed water from the bottom of the caliper and cylinder bores. Do it right, once, and in theory it is good for a decade and good to prevent corrosion.

Once water is removed from a DOT5 system it will not absorb more water. An advantage for long term storage and infrequent maintenance.

DOT3 & DOT4 glycol based fluids absorb moisture on a molecular level and do not separate water as efficiently as the silicone based DOT5. The water remains mixed in glycol fluids, in a proportion directly related to ambient water vapor pressure (relative humidity).

Higher humidity promotes more water in the glycol. Florida summers are not kind to glycol storage.in a brake system. The tropical high humidity moves into the glycol quickly. The location moisture is also difficult to boil out with use.

Low humidity promotes near zero water absorbtion into the glycol. Heat cycling glycol in a dry low humidity environment more easily removes water from glycol (it boils out quickly in small quantities). Live, drive, and.store in the dry desert or the cold north winter and there is little humidity or water vapor pressure to promote water contamination of the glycol.

Modern glycol brake fluids rival and exceed the boiling point of silicone fluid, when the glycol is dry. Flush the glycol before each race and glycol has a performance advantage. Flush the glycol every few years and it loses.it's performance advantage.

Tho environment concern was mentioned as cold, dry, winter storage. Glycol is less trouble in this environment, compared to Hawaii. Silicone has less of an advantage in this dry environment.

Few people misrepresent their experience with brake fluid, although few mention the environment. Get details on environment and use, and you will become more comfortable with validation of experiences related with brake fluids.

In a high humidity environment water can separate from the glycol during storage and promote localized corrosion. An owner needs to heat cycle the glycol or flush it on a regular basis to prevent excessive contamination of water into the glycol. Drive. Drive hard at times to heat and boil out the water in the glycol. Maintain the system like it is a hobby or race car. Flush the fluid every few years in low humidity environments, and more often in high humidity environments.

buns

Since the brake fluid isn't circulating through the system, and perhaps may move an inch before it returns, I can't see how this would benefit the fluid in the middle of the car, nowhere near the heat.

Just wondering.

63 340HP

The glycol moisture migration or diffusion is greatest at air/glycol interface locations, caliper and wheel cylinder piston seals. The mobility of moisture in glycol is from high to low concentration, from seals to line centers. Boil out moisture at the high concentration locations on a regular frequency and the moisture in the entire system is minimized through lowering the moisture gradient promoting migration in the glycol.

This moisture elimination is not possible during storage, where silicone based DOT5 excels as a corrosion inhibitor.

marieselden

DOT 5.1 should be ok

GTOguy

63340HP, very, very well put and I agree 100%.

davekp78

So how do you boil out the moisture?

rfn026

davekp just raised a very important point.

First - water boils at 212-degrees. Most of us never get our brake fluid that hot.

At Sebring my calipers only get to 300-degees (the rotors get to 1100-degrees). That's coming down from about 150 mph for turn 17. You'll never get to that temperature on the street.

Now - If you do manage to get your fluid to 212-degrees the moisture will boil and the pedal will go to the floor. That assumes you have a fairly high moisture level. Moisture turns to steam at 212 and steam is compressible. The moisture (steam) will not leave your brake system.

The only way to get the moisture out of the braking system is to flush it out.

Thanks Dave. I had never really thought about this before.

Richard Newton
Brake Temperatures

wmf62

like this....

Bill

63 340HP

Excellent reference and a great point that fluid temperatures in calipers seldom reach 212-degrees off the track (or off long grade descents). The calipers are a heat radiator, and designed to cool the fluid.

The missing concern is fluids flash and operate at temperatures higher than the calipers. The brake lines receive radient and conducted heat from exhaust and chassis proximity. Most people forget the system does not end at the caliper.

Take that IR gun or thermacouple and record line temperature at the frame and engine bay and you will get a shocking surprise. Master cylinder temperatures in line of sight from dull cherry red hot headers can reach 500+ degrees late in a 40 minute track practice session. The brake lines in the engine bay and under the vehicle can reach similar temperatures. The fluid is designed for 500 degrees for good reason.



Racing endurance events off road, failure of rear brakes on Volkswagen chassis vehicles is common. A climb and descent sequence can be a wake up call with no brakes on the way down. No brakes, so you find an uphill and stop. There is no pit 640 miles into the Baja 1000, so you must be independent and resourceful. Jump out and find the drums are only 300 degrees. Direct the IR sensor at the engine while crawling out and it is 600 degrees on the rocker cover (#$%@). Crawl back under and trace the IR sensor from the drum down the line back to the frame and find the hard line at 650 and slowly dropping (minutes after stopping). A little aluminum foil and duct tape at the next refuel stop helps keep the new brake fluid cool and keep going.

The next practice session with brake line shielding and insulation is much better. The next 250 mile race finds the car leading the class with a flat tire seventeen miles from the finish, losing four places to the flag. The next major Baja race has a similar record, leading until the driver gets lost in the last fifty miles, finishing second in class... with functional rear brakes. It gets better from there.

Amazing what a systems approach will find.

rfn026

This thread got me thinking about what I really know. Things change and I was wondering if I was up to date. My friends at Pagid sent me list of basics. It's HPDE oriented but you can't go wrong with their recommendations.

Depending on many factors including brake system, track, driver, brake cooling, fluid temp can get to over 500F. There are temperature indicator stickers available to put on the caliper. We suggest changing the caliper seals if the temperature exceeds 350F.

Most important is using a good racing brake fluid and bleeding brakes at least before every event. Pro Race teams bleed brakes before every session.

Boiling brake fluid develops gas bubbles in the calipers. The brake pedal becomes soft and pedal travel increases (gas is compressible). You can still stop the car by pumping the pedal, but effective modulation is gone. This is a gradual process with advanced warning. The damaged fluid must be completely replaced.

Correcting the problem requires improved cooling and / or may only require new or higher grade racing brake fluid.

Dry boiling temp should be > 300C (570F)

Wet boiling temp should be >200C (390F)

Brake fluid is hygroscopic i.e. it attracts and holds water molecules.

The importance of keeping fresh brake fluid in the system and regular bleeding cannot be overstressed.

Richard Newton