Technical Which Brake Fluid?

After a complete disassembly to clean and paint the T roadster I built about 12 years ago it's now going back together. I'm tempted to use DOT 5 fluid to have the advantage it has of not being an effective paint remover. I do plan to use care no matter what fluid I use, but somehow some tiny amount will get on some painted surface. The brake system consists of a CNC brand master cylinder, normal 3/16" steel lines and AN-3 braided hoses, and GM Metric calipers. I left the lines together where I could, but had them all off the car in sections that could be removed easily.

Since some of the reading I've done talks about the perils of mixing DOT3 and DOT5 fluid, just how drained/cleaned does a system have to be to consider going with DOT5 if it had DOT3 in it before? I don't want to create a problem by refilling a basically "dry" system, but realize there's always going to be some microscopic film of the old fluid on the inside of lines, calipers and the master cylinder no matter how long it has drained. Some even questioned if the two had an adverse reaction if intermixed. For the sake of a point of reference, I'd doubt of the total DOT3 that remains in my now reassembled brake system would be more than a tablespoon or two.

Why do I have the feeling this is going to be like a "which oil is best" thread?

Lynn

 

If I was starting new I would use silicone fluid DOT 5 works for me. Otherwise I would go back in with what it had in it.

 

So...porknbeaner....is my completely drained but admittedly not flushed brake system close enough to "new" that you'd consider refilling with DOT5? I've read about people flushing with denatured or isopropyl alcohol and then going with DOT5 brake fluid, but I think I'll just go back to DOT 3 or perhaps DOT 4 before I'd go that route. If flushing a system with alcohol I'd have the same question regarding how flushed out is complete enough? I wouldn't want a few tablespoons of alcohol left in the brake system either.

The older I get the harder it is to make decisions, no matter how seemingly simple they are.

Lynn

 

At work when we flush and convert a system to dot 5

I drain the reservoir and clean it out as best I can

Then fill with dot5 and bleed the system completely untill only dot5 (purple fluid)
Is coming out of the bleeders, time consuming and a bit messy but I have never had an issue going forward from that.

If you have access to a pressure bleeder or a vacuum bleeder will be quicker to get done.

On a side note if you have disc brakes I have opened the bleeders and pushed the pistons back on the calipers before starting to bleed the to make the process a bit quicker

 

I had new master cylinders fail from damaged seals and brake hoses swell in a way that the brakes did not release any more on 2 cars from using DOT 4/5 instead of DOT 3 in the last few years, so make sure all rubber components in your brake system are suitable for use with DOT 4 and upwards brake fluid...

 

Dot 3 and 4 are the same fluid base they are interchangeable

Dot 4 has a higher boiling point then dot 3

That’s why manufacturers say you can use dot 4 in a dot 3 system without issue and can use dot 3 as a “top up” or in an emergency in a system that calls for dot 4

You can also get super dot 4 fluid it has anti discolouring properties in it so it won’t go piss yellow or black as fast as regular dot 3/4 fluids.

Dot 5 is good fluid in racing applications , if you live in cold weather one issue with it is it does not hold water in suspension in the fluid, water will collect and can potentially freeze in extreme cold temps.

 

Well if you leave it open for a bit the alcohol would evaporate. But if it were me I would just go back with what I had and be real careful with it when I was pouring it.

That is good to know

 

This is true, happened to a bottle of Crown I left out last night.
After a lot of research and post reading I am using DOT 5 in my new brake system.

 

I once heard of a guy that was addicted to brake fluid...but they said he could stop anytime!

 

People who are serious apparently change brake fluid after every race.

Anyhoo DOT 5 is in a lot of ways a pain in the ass and costs 30 bucks a quart t' boot. Use a high quality DOT 3 and flush it every couple years. Remember the DOT boil points are minimum specifications, the manufacturers exceed these numbers by quite a lot for the good stuff.

Everybody sez "Conventional Brake fluid absorbs moisture!!!!"

And ... you know why? Because it's designed to! If it didn't, the old fluid in old style open systems would freeze solid in cold weather and boil to steam at high temperatures, either situation meaning no brakes at all.

 

DOT5 is not hydroscopic and as such does not absorb water as it is silicone based, originally introduced to give higher temperature performance over DOT 4 fluid. DOT5 fluid also has another advantage, it does not damage paintwork.
Whatever you do, don't mix DOT 5.1 (Glycol-based) with DOT 5 (Silicone-based) or any other DOT fluid for that matter due as each having different boiling points. The minimum boiling temperatures of brake fluid as per Department of Transportation (DOT) are:-


I used methylated spirits to flush the lines of brake fluid beforehand and replaced all the seals and cups. I know people who replace their fluid every year as a matter of course.

 

I have done several cars switching from dot 3 to silicone, I drain all of the dot 3 out I can from the lines and wheel cylinders and master cylinder. I fill up the master cylinder with silicone fluid and start the bleeding process. I'll run 2 master cylinders of silicone through then fill the master cylinder for the last time. I did this to my Sedan Delivery you see in my avtar. It has power discs on the front and drums on the rear. Been 25 years plus and many miles with no leaks or problems. Also did my family's 1948 Packard in the early 70's the same way and the brake system is still working without any problems. I used a good quality hydraulic brake light switch and it is still working with no leaks.

 

I pulled the trigger and went with DOT5 brake fluid. I filled the system today. Since the reservoir is the highest point, I just kept it full, opened all the bleeders, and let the lines and calipers fill via gravity. As I got a bit of fluid coming out at each bleeder I closed them and will now follow up with someone helping to do a proper bleeding using the pedal pressure. I was pleased that the very first sign of the DOT5 at each bleeder was nice and purple.....not cloudy or anything that would suggest it as being contaminated by any slight trace of DOT3 that I feared might be present after the system was drained. Just to be sure, I plan to flush the system with another full can of fluid just to make certain remaining traces of DOT3 are gone.

Thanks all for the replies. I didn't do an actual count of replies for or against the DOT5 fluid, but I got the feeling a number of people have successfully used and/or switched to it, so I felt confident enough to give it a go.

Lynn

 

DOT in my 32 and my old Shovel now . Only issue I have ever seen is brakes feel a little soft for the first few stops , once the fluid reaches operating temp , I can not notice any difference . If I trust my life using it while riding on 2 wheels , I suppose it is good enough for the top shelf in my garage

 

Interesting that HD are the only OE manufacturer that I know of that routinely used Dot5 silicone fluid.
Almost every body else used Dot 3/4/5.1 . Probably because it was cheap, reliable and safe. There are a few odd brands out there that use mineral based fluids (Rolls, Citroen etc) but the majority keep it simple.
If you are unlucky enough to spill glycol based brake fluid, don't wipe it - flush with plain water -fluid gone and paint safe.
I will go with the mainstream brands every time . Dot 3 / 4 and 5.1 can be interchanged, but keep an eye on the boiling point for any performance vehicle.
Garpo