The Stovebolt Site Forums The Shop Area Driveline Dot 3 to Dot 5 Brake Fluid

I working on a resto-mod of a '55 1st Series Chevy Pickup. I replaced the original master cylinder with a dual master cylinder and the new system has disc brakes in the front and drums in the rear. Several years ago when I first got the master cylinder I used some Dot 3 brake fluid to bench bleed it. It was a few years later when I finally got all the lines done and was ready to fill the system with fluid. I decided at that time to use Dot 5 fluid, having forgotten for the moment that I had put Dot 3 in the master cylinder previously. After filing the system I noticed the color of the fluid changed and remembered I had put some Dot 3 fluid in the master cylinder several years ago. The master cylinder also started leaking at the front where the push rod goes into it. I am going to either replace or rebuild the master cylinder. I also plan to flush or blow out all the lines before I put the new master cylinder back on. Should I be worried about the wheel cylinders or disc calipers? If I clean out the master cylinder and rebuild it and clean all the fluid out of the lines and wheel cylinders/calipers, should I be good to go with refilling the system with Dot 5? If not, what all needs to be done to make sure the new fluid doesn't get contaminated again?

According to what I just read, you can't mix DOT 5 with anything else, as it's silicone based, as opposed to glycol based. It was developed for use in applications that may have high moisture intrusion, such as military.
To confuse things more, DOT 5.1 is glycol based, so could be mixed with DOT 3 or DOT 4.
I would flush everything out and use DOT 3 or DOT4. Even my wife's '13 Escape specifies DOT 4. I'm don't know of any vehicles that specify DOT 5.
The latest isn't always the greatest.
This is an interesting read.

Kevin, that is indeed an interesting read and reveals very confusing terminology. One would think a new glycol based fluid should be named 4.1, rather than 5.1 which suggests it is silicone based. To add to the comment about silicone being lighter than water I would like to relate my only experience with DOT 5. I know that members of this site have used DOT 5 successfully but here goes anyway. A neighbor began working on a 70s RV he purchased and he borrowed some of my tools to go through the brake system. The previous owner had converted the brake fluid to DOT 5. When he disassembled the wheel cylinders they all had water and horrible rust pooled at the lower dependent portions of the cylinders, requiring replacement. This vehicle had sat outdoors for several years unused. But this outdoors was Phoenix, AZ. I don't know how that much water got in there, whether it had been driven through water or whether our winters are moist enough to promote condensation in the cylinders. I don't know whether it would have been worse with conventional brake fluid or not. I came away thinking that I would stick with glycol based fluid and just bleed the brakes once in a while to remove any moisture.

To try to answer Bill's original question and if I understand you correctly, you may have to rebuild all of your cylinders unless you know the rubber parts are compatible with DOT 5 silicone. Someone can chime in with how to flush DOT 5 from your brake lines as I don't know how you do that.

I think a standard flush should get everything out of the lines. If the DOT 5 is lighter than water (and presumably lighter than the glycol based stuff, I'd assume that the DOT 5 would rise to the top of the cylinders and be flushed out of the bleed port. If that were the case, you'd see DOT 5, then any water that might have accumulated (not likely for a recently filled system), then the glycol based fluid. But never having done that, I'm not sure.

That also assumes that the cylinders don't need rebuilding. With no real way to tell if the seals were compatible, I'd rebuild everything to be on the safe side. Brakes are something you should be able to depend on. "That should work" shouldn't apply to brakes.

Bill,

Here's a great discussion of your subject on the Chevytalk forum:

https://www.chevytalk.org/fusionbb/showtopic.php?tid/356278/

Notice the give and take of information in this discussion of brake fluids. That's the hallmark of a highly effective information sharing forum.

Notice also that fluids ARE compressible despite what various "Experts" assert and despite what our high school and college physics teachers told us.

Like you I think a tandem MC is a worthwhile safety upgrade and I adapted one into my '36 Chevy PU, the first year Chevy used hydraulic brakes. There is some accumulated grime in the photo because this '36 was restored in the early 1970s and has been to a a few shows where it did fine but is mostly driven and had accumulated plenty of mileage in the last 45 years or so.

Ray W

I switch back and forth with my 1960 MGA with no issues. I know that 3 and 4 will eat paint but 5 will leave fish eye in the paint if it gets on the metal and then you paint and is very difficult to remove. I had my master cylinder leak and it was near an engine vent and the fluid blew onto the left front fender and no amount of washing would take it off. I know this has nothing to do with the current question but at one time it was not recommended to use Dot 5 in corvettes it made the brakes spongy.

"I know this has nothing to do with the current question but at one time it was not recommended to use Dot 5 in corvettes it made the brakes spongy."

Good morning Ron.

If you look at the Chevytalk discussion the "spongy" issue is addressed.

https://www.chevytalk.org/fusionbb/showtopic.php?tid/356278/

Ray W