View Full Version : Brake Fluid Types

12-08-2015, 07:00 PM
I had a little epiphany the other day about brake fluid. This might be common knowledge to those with more experience, but I never had this knowledge. A little background:

A couple of years ago our noble Treasurer helped me bleed the brakes on my van after getting the master cylinder rebuilt. All worked well but if I was at a stop holding pressure on the pedal, the pedal would slow fade to the floor. A quick pump brought it back up. In the end I ended up bleeding the entire system to rid the issue.

This past August I experienced a similar behavior in my daily driver. I replaced the front calipers, bled the system, and very similar behavior. This has system a vacuum boosted master cylinder.

My epiphany was this: mixing synthetic with non-synthetic brake fluid. To confirm this I used non-synthetic fluid after changing the front calipers on my GF '77 Mercedes. No brake fade.

I do understand there are other considerations: DOT3/4, DOT 5 vs anything else, etc....

It brings up a question, do you prefer synthetic or regular brake fluids? Since synthetic seems to be on every shelf it makes sense to use it out of convenience, however perhaps these older components don't like it? Is synthetic the "ethonol" of brake fluid? Do we need to worry about the seals on the master?

12-08-2015, 10:25 PM
I asked some questions about changing to synthetic over DOT brake fluid and this is the answer that was sent my way. I didn't switch. Larry

If you do decide to convert to silicone fluid, it should be done as part of a total brake system overhaul, with freshly rebuilt or new calipers, wheel cylinders and master cylinder. Silicone fluid should not be added to a system that contains even small amounts of glycol fluid or contaminants. Merely bleeding the system is not enough, as there will be pockets of old fluid and sludge that will not bleed out. Silicone fluid tends to concentrate any residual glycol fluid, moisture and sludge into slugs instead of allowing their dispersal throughout the fluid, as does glycol fluid. This can lead to relatively severe but localized problems, rather than the more general system deterioration experienced with old moisture-laden glycol fluids. This may be a factor in reports of leakage when silicone fluid is used in non-rebuilt systems that had been operated with glycol fluid. A "new" system full of silicone fluid will require very little maintenance for years.

12-09-2015, 12:29 PM
I don't think you'll need different-type rubber bits, which is true with the Ethanol example, but you do need to make sure (as Larry indicated) you have purged all traces of DOT3-like fluid from the system. A full dis-assembly of every part is needed for this. And a flush of all the lines. Non-synthetic fluid is hygroscopic and synthetic fluid is not. So a synthetic-filled system will not attract moisture, which is the leading cause of brake component failure. AlaRust.

12-09-2015, 01:30 PM
I found this thread at the link below interesting...and at the bottom a list of other threads on the topic as well. I will have new lines and new MC, but the new hoses and wheel cylinders have had new Dot 3 in them...trying to decide if flushing them and going to Dot 5 (because I don't want to ruin my paint job) is a good idea. Some say no, some say it can be flushed successfully. Good luck!


12-09-2015, 04:29 PM
It's a relatively simple chore to take the wheel cylinders apart and really clean them. We used to do it whenever we needed, as it was a much less expensive alternative to disassemble and hone than to remove and replace. Just takes time.

12-09-2015, 08:48 PM
There are advantages to switching and why I almost did, is to eliminate the paint damaging attributes of the regular fluid. Everything I read said to make sure you flush the whole system with alcohol. If you need help, some of our imbiber members, if we have them might volunteer the liquid. Kidding. But, I would suggest doing that step. Larry

12-10-2015, 12:16 PM
Larry - I'm sure they meant Isopropyl Alcohol. Way cheaper than Vodka (I would guess). :rolleyes:

12-10-2015, 05:21 PM
Well, since I have somewhat mixed system (I'm sure there is some DOT remnants) looks like I have some cleaning to do. I have revamped my vacuum brake bleeding setup recently so I guess it is about to get a work out.

With all of this trouble I will make the switch to synthetic for the long haul. I'll let you know how it goes... :doh:

12-10-2015, 10:32 PM
Take a look at more of the web sites on changing the fluid. There are quite a few that changed and went back because of poor brake feel. Spongy, soft, etc.

Roger, I don't drink anything more than a beer every once in awhile so that probably wouldn't work either. Most likely foaming at the master cylinder.:BEER:
Sounds like something we could try at our next tech day. Yahoo

12-11-2015, 01:44 PM
Most well written article I've seen yet...


12-11-2015, 06:09 PM
After going over that information I went and got a beer. :WHATTHE: Crap, Einstein didn't go through that much effort in his theory of relativity. My head hurts. One thing for sure, I am not the intellectual type. For me, it comes down to two questions. Is it better? Is it better enough to make me do the work. Man I love this club. We have the most remarkable group of car enthusiasts in the country. Need one more beer. :BEER: And I don't drink. :3g:Larry