I thought this would be easy, but the more research I do, the more difficult it becomes! I welcome your opinions... What I learned: - All brake systems will absorb moisture. It will work it's way into your system by permeating thru hoses, seals, and thru an open master cylinder cap. Of course, some climates are worse than others.....there's lots of water here in the Northeast. - Glycol-based DOT 3 and 4 fluids will absorb this moisture, and will hold it until they get 3-4% moisture content...at which point the anti-corrosion additives lose their effectiveness, and corrosion of your system begins. - SYSTEMS USING GLYCOL-BASED FLUIDS SHOULD HAVE THE FLUID CHANGED EVERY FEW YEARS! - I despise the fact that glycol fluids will ruin my painted components! -Silicone-based fluids will not mix with water that enters the system. Instead, accumulated water will move to the lowest point in the system and accumulate there.....and which point it could cause spot corrosion of wheel cylinders or brake lines. - Water accumulated in a caliper or wheel cylinder, under hd (hot)conditions, could boil and vaporize, severly affecting braking performance. - It appears that many hp and race applications recommend NOT using silicone-based fluids. Synthetic DOT 3,4,and 5.1 - A Google search for 'synthetic brake fluid' brings some promising leads. Some seem to be just marketing gimmicks, but some look promising. - Anyone have experience with Synlube? http://www.synlube.com/brake.htm - Are there synthetics that will NOT harm paint? Any experts out there?
Unless every brake component that will see fluid is new or completely clean, don't mess with DOT 5 (silicone)...it has poor compatibility with glycol-based residues. Personally I don't have experience with synthetics as the OEMs aren't quite there...yet. Unless it's a race car that requires high "dry" boiling point, I'd stick with DOT 3 or 4. I'll try looking up more on synthetics when I get to work tomorrow (fuel/brake sys engineer). I like the looks of 5.1, which is non-silicone based, but need to read up on that some more as well. Did a quick scan of this article, looks like it's worth the read: http://www.stoptech.com/tech_info/wp_brakefluid_1a.shtml Devon
Thanks Devon! I'd appreciate any input you have on the synthetics. The article you linked to is a good read, and I'd also recommend it. Do the manufactures (Ford, GM, etc) specify that the brake fluid be changed at regular intervals? I never realized it was a service requirement, until now.
i remember reading somewhere on this board that rebuilt calipers and master cylinders are assembled using dot 3 brake fluid as a lubricant. will that minute amount be a problem if you use synthetic fluid in the system? I have been trying to decide also and thought it was a no brainer since i was replacing every component of the braking system. o No:
From http://www.aa1car.com/library/bfluid.htm though I can't validate the OEM comments from home: GranSportSedan... All of the DOT 3&4 synthetics I've read up on are compatible with standard DOT 3&4 fluid. I'm not surprised, as the aftermarket needs to make sure the average joe would have no issue filling up with synthetic when there's remnants of the old stuff (same applies for engine oil). Otherwise, nobody would buy synthetic. It's the Silicone based fluids that will not tolerate any remnants of the standard brake fluids, you need to start from scratch with that stuff. Devon
DOT 5 is military spec fluid, so we use it in our GM vehicles destined for Military usage (ie, H1 Humvees, the old CUCV, etc). It is not hydroscopic, which means it does not draw water out of the air, so that should result in less corrosion in your brake lines. And, you are correct, it does not mess up automotive paint. You can't mix the two types of brake fluid so you have to install it in a dry system or bleed the DOT 3 fluid all the way out. Some of the more "anal" brake engineer-types would even suggest that you replace every rubber component that has ever been exposed to the DOT 3 (hoses, o rings, m/c, etc) before you put in the silicone, but I think that is a bit extreme. Now, for my personal experience: I ran silicone fluid in my Chevelle for a number of years and for whatever reason got to the point where I had to put a new master cylinder on it at the end of every season. I finally went back to the traditional brake fluid and that problem went away. I still can't say for certain if the two were related. Conversely, I have run it in my open car trailer, which has surge type brakes, and have never had a problem. My $0.04 worth - K
A little more reading and correspondence today. Looks to me that the only benefits of silicone (Dot 5) fluids are: 1. Higher boiling point that Dot 3 & 4 2. More stable viscosity over the operating temp range 3. Not hydroscopic (won’t absorb moisture) 4. Compatible with most standard brake system materials 5. Paint-safe Cons are: 1. Higher affinity for air molecules (contributes to spongy/soft pedal) 2. No-go for antilock brakes due to incompatible viscosities 3. Not hydroscopic (won’t absorb moisture), so any water in the system stays separate and can cause corrosion 4. Said water in system boils at low temps contributing to spongy/soft pedal 5. Incompatible with Dot 3 & 4, shouldn’t be used with anything but dry, virgin systems (mixed opinions on this) Synthetics: From a couple folks at the OEMs I spoke with this morning, the only negatives are cost, and they're still harmful to paint. Performance is comparable to or greater than standard DOT fluids. Best bet: Go for DOT 5.1 or better yet, synthetic DOT 5.1. Avoid silicone based DOT 5 unless you have a show car that for some reason might get messed up by standard brake fluid. My $0.02... Devon
Thanks Devon! I guess I have to accept the fact that I need to regularly change the fluid in my cars. With that being the case, it would be nice to have a fluid that is paint-safe. I should add that DOT 5.1 is not silicone-based, and should not be mixed with DOT 5 silicone fluids. DOT 5.1 has an increased temperature rating of a DOT 5 fluid. But is chemically more similar to the DOT 3 and 4 fluids.....confusing isn't it?
