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been hearing that dot 5 silicon base brake fluid is not corrosive as all other[ dot 4 and 3] anyone have insights on this?  im considering using it on my 46 chevy truck cause once the wheel cylinders leak aliitle the corosion is bad  im in process of replacing all cylinders and am considering this dot 5    it is important to get all the old fluid out i understand which is somewhat a pain but if it stops the corrosion its worth it to me   but i may be missing something so id appreciate any wisdom on this     thanks   bought a master cylinder from a very popular classic chevy supplier on line    little over a yr ago  noticed it dripping and tore into it  it was pitted and apparantly was patched and sold to me  w some filler  the filler lasted a yr and its to deep to hone out  so again everything got corroded  i guess the warning when i bought it was industria argentinia stamped   anyway thanks ahead 

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I've been using DOT 5 brake fluid in my classic cars since the early 80s. I've never had  problem with it. I do always add it to a clean system. Usually because I replaced all the hydraulic hoses, steel lines (now I replace those with Cunifer) and rebuild the master and wheel cylinders at the same time.

 

I love the fact that my classic cars have working brakes for years. Not the typical DOT 3 systems that form corrosion in just a few years causing pistons to seize in the wheel cylinders with that white crap. Who actually flushes their DOT 3 systems every 2 to 3 years?

 

Some people have issues with DOT 5. I did not understand why until I saw a guy "help" bleed a brake system. He pumped up the master by quickly and forcefully pressing the brake pedal to the floor about 20 times everytime he wanted the other person to open the bleeder. He was making foam in the master cylinder better than the food industry making whipped cream! Where do people get these ideas? Not from any shop manual I've read....

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I installed silicone in my 1937's brake lines about 25 years ago, and essentially left it alone.  Last summer I found a small leak in one wheel cylinder and replaced the cylinder.  That's about the extent of my problems using silicone.

 

I have heard complaints by some people with hydraulic brake light switches, that the silicone manages to leak into the switches.  I don't know if this problem is widespread.  My car has a mechanical brakelight switch that has no connection to the hydraulic lines, so of course it wouldn't have been a problem with me.

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I had a supposedly knowledgeable service guy rejuvenate the brakes on my '64 Chrysler last fall - I believe it originally had Dot 3 fluid in it. In a fairly short time the fluid drained itself out of all the wheel cylinders, leaving me with no brakes. Here's what I suspect is going on: the car had sat for 10 years and the original brake fluid had leaked out and likely the wheel cylinders needed to be redone or replaced. The brake lines were replaced 10 years ago so they are good. HOWEVER, without having the system completely flushed of the old Dot 3 fluid and at least putting kits in the wheel cylinders, what I got was essentially nothing. Now I have heard that Dot 5 will find any exit it can, and with a compromised system that appears to be what has happened.

Now my question. What is the best procedure to flush ALL of this out and start over to get the brakes working properly and safely?

Thanks for your help.

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I've put Dot 5 in Every vintage car I have and don't even let Dot 3 near an old car I have unless it's a car I haven't done a conversion on and just want to get it moving and stopping.  Almost every car I buy ends up getting a full brake job and usually tires, first thing as they usually have been sitting and even though some of the brakes on them seem OK are usually junk when I start getting them apart. 

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My Dad converted to dot 5 about 30 years ago in a 57 Tbird I now owns. No issues except the brake switch issue he replaced about 10 years ago.  I put dot 5 in a 70 Corvette about 5 years ago after years of issues with air in the system. Redid everything but the lines and have had no issues. Plan to put it in a 66 impala this spring after a rebuilding everything. 

 

Tom Muth

CIncinnati, Ohio

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20 minutes ago, Spinneyhill said:

Just note that DOT 5.1 is a development of DOT 4, not DOT 5.

 

If DOT 5 is so good, why don't car makers use it in new cars?

Because it does not work well with ABS braking systems.

Also 5.1 is the highest temperature rated Glycol Ether brake fluid...higher than DOT 3 or 4 fluids. All 3 absorb water quickly.

DOT 5 is a silicone based fluid not associated at all with DOT 3, 4 or 5.1

The military uses DOT 5 silicone fluids.

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22 minutes ago, Spinneyhill said:

Just note that DOT 5.1 is a development of DOT 4, not DOT 5.

 

If DOT 5 is so good, why don't car makers use it in new cars?

 

I'm also a fan of Dot 5, maybe new car makers don't use it because it is more expensive and what they use gets them through the warranty and then they can sell you brake parts.

 

I have converted 3 cars so far with no problems. I rebuild/replace all cylinders and on the ones I didn't replace the lines on I flushed with spray brake cleaner and let it evaporate before filling. Last Summer I replace some rubber brake hoses I should have replaced 20 or so ago when I rebuilt the brakes system with Dot 5 but didn't. I pulled one brake cylinder apart to make sure all was good since I had to bleed the brakes anyway. Looked like new just the way it was when I put it on.

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16 minutes ago, Spinneyhill said:

 

How?

 

And how much more does it cost?

DOT 5 When agitated rapidly (such as the on/off cycling of ABS valves) silicone-based brake fluid foams excessively. Foaming causes the same behavior as if there were air in the lines...so once ABS engages, you're suddenly not stopping.

Up to 4-5 Times as expensive as DOT 3 or 4!

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3 minutes ago, c49er said:

DOT 5 When agitated rapidly (such as the on/off cycling of ABS valves) silicone-based brake fluid foams excessively. Foaming causes the same behavior as if there were air in the lines...so once ABS engages, you're suddenly not stopping.

Up to 4-5 Times as expensive as DOT 3 or 4!

 

OK, thanks very much for that. I am surprised they haven't managed to find a formulation that doesn't foam.

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I have done several conversions for the collections I have worked for, and Dot 5 is the only way to go for me. As mentioned before, You have to flush the system of all Dot 3 fluid. If I am not replacing the hard lines on the car, I remove all wheel cylinders, plug the ends of the hoses and fill the lines with denatured alcohol, and let it sit overnight. Pull the plugs and let it drain, then blow out the lines with compressed air. I automatically install new or rebuilt wheel cylinders and Master cylinder as part of the job. A lot of people don't know to do this, but new master and wheel cylinders out of the box are not ready to use. They are coated with an anti-corrosion goo, like cosmolene that doesn't help the workings. I take them apart, clean all of the steel parts with lacquer thinner, Paint the exposed cast iron parts and clean the rubber parts with denatured alcohol. Coat the inside bore of the cylinders, pistons and piston seals with the type of fluid you are going to run the system on--if you are staying Dot 3, coat everything that moves with Dot 3, if Dot 5, likewise. 

The person pumping/holding the pedal has to make slow, deliberate strokes, not rapid "jackrabbit" strokes as you do with Dot 3. As mentioned before, pumping fast induces air into the system.  

 

Cars my mentor restored in the 1980s  still have great brakes, done with Dot 5. 

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Comments on the inadvisability of using Dot 5 in a 50's system containing a Hydrovac, or similar vacuum assist braking system. Theoretically the scenario is that small amounts of fluid can be sucked into the engine, and because Dot 5 is silica based, and an abrasive, can cause premature engine wear. Is there any real word evidence of this occurring?

 

When I restored my 1955 Studebaker in the 70's Dot 5 was new, and I used it. The car has been driven sparingly over the past forty years and has sat for about two years. Recently got it running, which was no problem, but no brake peddle. A check of the fluid level shows the master cylinder to be full, but pumping does not restore the peddle. I may have to resort to pressure bleeding, but sill unsure if this is the answer. Is there a significant reason to return this system to Dot 4 if a rebuild is required? 

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I signed up to the Dot 5 group back in 2006 when we finished restoring our MGBGT.  Believe me, the first time you spill some Dot 5 on a fresh paint job you'll be glad you went that direction.  Happened to us when were installing the brake lines on the finished restoration.  Forgot to include a washer and it poured out, dripping down our freshly painted firewall.  All we needed to do was wipe it off.   Regular stuff makes a pretty good paint stripper and that was one of the reasons we went with Dot 5 to begin with.   So - with now over 30,000 miles of the car, we have never had to bleed the brakes and everything is still as new - clear fluid, no rusty parts, etc.  This little car gets driven, including mountain roads, lots of twisty-turnies going places were MGs like to go.  Straightening out curves is our favorite passtime.

We live the MG motto - Safety Fast!

Terry

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I have been converting to dot 5 for the last 12 or so years. Occasionally, you will have a problem

with NOS pressure switches sticking.

I have often wondered, If you buy a car and the brakes have already been done, is there a surefire

way to tell whether it is dot 3  or dot 5. 

When I convert . I usually stamp the master cylinder cap or tag it 

Dennis

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Put your finger in the fluid.  Definitely different feel as well as smell.  Put a drop on a dirty surface.  Silicone will migrate very fast.  Once you'll used silicone, you'll know the difference.  Usually a purple color as well.

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  • 3 months later...

I switched to DOT 5 and found that all the following to be true on my vehicle.

Compressibility — Silicones have a poor bulk modulus. In practice, this means they can compress up to three times more than conventional fluids when pressurised in a brake system. This results in a “spongy” feel to the brake pedal, together with long pedal travel. 

 

All new master, booster, wheel lines and wheel cylinders done at same time as switch over to DOT 5

Any suggestions as to what I can do to improve the situation?

Thanks

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The pedals have always been good on my cars.  Are you sure there is no play in any of the linkages.  I know my Hudson has alot of pivots and that little play in each really adds up.  Did you adjust everything up in the wheels then use the system a little with a few kind of hard stops,  then readjust the wheels?  I know my 49 Chevy Pickup had a poor pedal until I readjusted everything after it got all squared back up in the wheels after being all apart on the garage floor.

Just some things I have experienced and learned to check for first.

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It's air.

 

This stuff is near impossible to bleed. Once bled, Its wonderful. No more perishable corrosive stuff eating your cylinders.

 

Try this. Pour some in a jar. Let it set a while, maybe even overnight. Look at it. Look at it real good. Clear? Good! Now pour some more fluid in the jar and look again. Get out your magnifying glass, or better yet your loupe. See those little things? those are BUBBLES. They aren't floating to the top very fast, are they?

 

This is what happens when you pour more fluid in and then bleed. Those little tiny bubbles get bled back INTO the system.

 

If you put this in a system, fill it as much as you can and let it set a long time. Overnight would be good. Next morning, gravity bleed if you can. If not, use a vacuum bleeder. Pumping it up and squirting it in the traditional manner is not going to work. When the reservoir gets close to empty, refill it and walk away. Come back later, much later. Maybe tomorrow. Bleed until you get low again. If you have to add fluid, walk away.

 

The jar experiment will give you some idea how long it needs to sit after you add fluid. You want it to be clear for a long time before you attempt to bleed. The smallest bubbles seem to take the longest time to come out, and are the hardest to see.

 

Once bled, and you have a good pedal, drive the car and hope it shakes any remaining air loose. One final quick bleed at each corner a day or two later may make it a little more solid.

 

30 years ago, Johnsen's was easier to bleed than some of the others (Sta-Lube, etc). I have no idea which one is best now, but the brand can make a big difference.

 

This stuff is a giant pain, but a restored car really is the ideal use for it. Many of our cars don't even try to keep the outside air away from the fluid. Once its in there and bled, it lasts a long time, and doesn't sit there sucking water out of the air like DOT3/DOT4 does.

 

DonHo.jpg

 

 

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Like Bloo says, do not pump the pedal during bleeding. Nice slow strokes! I recently saw a person "help" bleed the brakes on a new master cylinder installation. He pumped so rapidly, it sounded like a machine gun. And, the firewall flexed! This would not have been good if he was using DOT 5. Since he used DOT 3, it actually worked. But no way is that proper bleeding procedure! 

 

I've never had a problem with low pedal and DOT 5 on the many cars I have done the changeover on for the last 35 years. I use slow pedal strokes if using two people or a one man bleeder bottle (see most any shop manual), or I use a mighty Vac. Gravity also works on firewall mounted masters.

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Silicone since 86, no issues except less maintenance.

 

If you think about bleeding, you'll realize there is no point in pumping the pedal.  One stroke and hold is all that needs to be done.  In other words fill the system with one stroke of the master cylinder at a time, a few feet of line.  Pumping implies the master piston can return and allow fluid from the non-pressurized reservoir to flow into the system before the air bubbles expand.  That isn't going to happen.  Everything just returns to the original volume.  After the bleeder is closed and the pedal returns, the system may now have a vacuum which the reservoir will gladly fill.  Pumping may have worked with older fluid, but wasn't necessary and definitely doesn't help with silicone.

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I learned something a long time ago while owning Metropolitans with the dreaded Girling brakes.  Air is almost impossible to get out of that system.  If you find you have a mushy pedal after bleeding the brakes,  cut a stick the length of the brake pedal to the bottom of the dash and pump up the brakes and hold it there overnight with the stick and the trapped air will be gone the next day.

On that note I use nothing but DOT 5.

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1 hour ago, Paul Dobbin said:

Used Dot 5 in a 1940 Ford pickup in 1984, still works fine.

Put it in a 1981 Chevrolet El Camino 5 years ago.  Works fine, except the brake light on the dash is lit all the time.  Black tape fixed that!

Yes, I found that to be true with old working switches and NOS BL switches.  I found a NEW switch at NAPA that judges fine and is not allergic to DOT5.  I run DOT5 in all my collector cars.

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