Brake fluid choice?

[Laughing Coyote]

I am ready yo rebuild the wheel cylinders on the Mercury that I'm restoring and I would like to get some input on what type of fluid would be best.  Dot 3, Dot 4, Dot 4 with silicone or Dot 5.  I've read Dot 3 can attract moisture over time and you will have to rebuild the wheel cylinder often.  Dot 4 is better and the silicone base prevents the moisture.  The Dot 5 is best if your not going to be driving the car as a daily, but the pedal is spongy.  Thoughts, opinions, etc.

Thanks

 

[Willie Wurke]

Use Dot 3      The silicone fluid has no lubrication qualities for the rubber cups in the cylinders.

 

In response to Spinneyhill

I don't think I am misinformed about this. I got my information from the Rolls-Royce engineering division in England

several years ago. I have Dot 3 in my Rolls. fronts only. The rears are mechanical with a booster!

 

  I tried silicone fluid in my Mk II Continental and was not satisfied. After sitting about a week I had to stomp on the brake pedal.

Once I broke it loose it was OK for the trip. It had a Treadle-Vac booster which doesn't have a piston with rubber cups as normal.

It uses a steel rod(plunger) with a leather seal.

Edited June 24, 2016 by Willie Wurke (see edit history)

[Spinneyhill]

Sorry Gentlemen. You both seem to have misconceptions. Read the Wikipedia article on brake fluid.

 

DOT 3 and 4 are both based on glycol ether. DOT 4 has a higher boiling point than DOT 3 and is slightly more viscous to cope with the higher temps. It also contains a Borate ester. DOT 5.1 is also glycol ether and borate ester based, with even higher boiling point. Silicon (DOT 5) has a similar BP to DOT 5.1.

 

Wikipedia says...

"Silicone based fluid is more compressible than glycol based fluid, leading to spongy feeling brakes. ^[3] It can potentially suffer phase separation/water pooling and freezing/boiling in the system over time - the main reason single phase hygroscopic fluids are used.^{[citation needed]}"

 

I don't believe that statement without better evidence. The spongy feeling ref. is to another web site - where did they get it from?

 

So based on all that, after a rebuild I would probably use DOT 5.1 fluid. If you do, stick with it. Do not mix DOT 5 and any other fluid. If you want to go to silicon, replace or wash with alcohol all brake lines and all cylinders. Wikipedia says the US army standardised on silicon in the 1990s.

[Joe in Canada]

You have to watch brake fluid with silicone as it will make some rubbers swell. Like the older rebuild kits were the rubber is not compatible with silicone. 

[auburnseeker]

In all the cars I have used Silicone, Probably around 10 over a 20 year span,  I have had good luck and never had a spongy pedal or problems with the rubber components.  Some were old and some were new. Some were patial rebuilds and some were completely brand new systems.  I wonder how much of the problems floating around with silicone brake fluid are really caused by the Fluid or by other failures of overlooked parts that didn't really pass the muster when the system was repaired or rebuilt.. 

[Laughing Coyote]

Thanks for the info so far.  The brake system has been totally disassembled and cleaned.  The rebuild kits for the cylinders are new aftermarket and not NOS.

[Marty Roth]

I have used DOT-5 exclusively on any total rebuild of a brake system - and have NEVER had a spongy pedal, nor any other problem with those systems. 

 

Partial rebuilds and repairs get an addition of DOT-4 since it will mix with DOT-3, and is less hygroscopic (less absorption of moisture, therefore less likelihood of corrosion in the system).

 

Brake systems should be flushed every few years, but hardly anyone seems to do that until they have problems. I flush with alcohol, and allow the system to "AIR-DRY" afterward. This seems to work best before refilling with Silicone DOT-5

 

Just my opinion, but it seems to work

[ply33]

From my personal limited experience, based on using DOT5 on one car which started with all new or fully rebuilt components in the braking system.

 

Cons:

• Silicone (DOT5) is harder to get properly bled. Any bubbles that are entrained in it, say due to careless pouring take a long, long time to come to the surface and release the air. If you attempt to bleed the brakes with air bubbles in the reservoir then you are just adding more air to the system that will require more time and effort to bleed out.
• It is a lot more expensive than conventional brake fluid, so if you run through a lot in bleeding (see issue with entrained air being slow to go away above) it can be more expensive than you think.
• Areas where it was spilled may have issues in painting at a later date unless you do a very good prep job.

 

Pros:

• Doesn't ruin existing paint if (in my case when) spilled on painted surface.

 

From hearsay:

Cons:

• May cause shorter life of hydraulic style brake light switches.
• May form silicon abrasives when burned in engine (due to defective vacuum type power brake booster). So an unnoticed power brake booster problem may lead to requiring the engine to be rebuilt. (Not an issue on my car but might be on a newer one.)
• There are issues with going to or from DOT5 in an existing system. Basically they have different densities and don't mix so you will end up with some place where one type of fluid is floating on the other and can't be bled out. Solution to that is to fully disassemble and clean everything and then start over.
• Disposal of used fluid may be worse. Apparently it does not break down in the environment as readily as conventional brake fluid.

 

Pros:

• Does not absorb water vapor from the air leading to reduced failure due to corrosion in metal parts of brake system.

 

My take away:

The brakes work fine with either type of fluid. And my pedal does not feel spongy.

 

But since I replaced all tubing with corrosion resistant Cunifer and sleeved all the cylinders with corrosion resistant brass, the reduced brake system corrosion with DOT5 is probably not something I needed to worry about (that was my main reason for going with DOT5). Were I to do it over, I'd go with DOT3 and simply plan on flushing the brake system every 3 to 5 years. And I'd have to learn to be more careful about spillage of brake fluid on painted surfaces.  Fortunately the spillage should only one time when initially bleeding the system and again during periodic flushing, so being careful about spilling is not an every day or even every year thing.

 

[Guest Skyking]

I just did a complete rebuild on the brake system in my Chevelle.  It was a powerless system.  I added power disc up front and used DOT 5.  The only problem I   encountered was getting all the air out.  I've learned a little trick that most Metropolitan owners use due to the English brake systems in those cars.  The stick trick!  

 

After I bleed the system out I still had a low pedal due to the tiniest air bubbles.  If you pump the brakes and place a stick between the pedal and dash and leave it overnight you'll have a hard pedal in the morning.  Somehow,  it works.  I've put on 300 miles and I have a firm pedal.

 

I also have DOT 5 in my '54 Metropolitan.  The only other problem I've had was the brake light switch.  They seem to fail periodically with the DOT 5.  

With the Chevelle the switch is separated from the fluid........

[Bhigdog]

I've used dot 5 in all my collector cars. My Corvette has had the same fluid for over 20 years, my 39 pick up for 17. No spongy pedal, no corrosion, no problems........Bob

[Tom99]

I put DOT 5 in my 1969 Plymouth Road Runner in 1989. The pedal is firm, not spongy, and the fluid is still as clear as water.

I drive the car about 20 miles a week.

[tomcarnut]

I just inherited my Dad's 57 T-bird he put dot 5 in over 25 years ago with no problems but the brake light switch. I have a 70 Vette which I had the system completely rebuild about 5 years ago with dot 5 and no problems or spongy pedal. Plan to put dot 5 in some other cars when rebuilding the system.

 

Tom Muth

[jpage]

One of the only drawbacks I've heard about DOT-5 silicone fluid was that it's viscosity can cause it to leak more easily that the standard DOT-3. Don't know for sure, but that's what I've heard.

Edited June 24, 2016 by jpage (see edit history)

[bill pritchett]

I have used DOT 3 synthetic brake fluid which is available at all auto part stores.  It is water resistant and compatible with regular 3 and 4.  It seems like a great compromise.  Also, I have a 54 MG with silicone brake fluid and got 25 years out of it and I just replaced it with new fluid of the same kind after a complete rebuild of system.

[poci1957]

30 minutes ago, jpage said:

One of the only drawbacks I've heard about DOT-5 silicone fluid was that it's viscosity can cause it to leak more easily that the standard DOT-3. Don't know for sure, but that's what I've heard.

 

I have a 1957 Pontiac with Treadle Vac and my Treadle Vac rebuilder told me that.  But I really wanted to use DOT 5 in my system and so far have been doing fine.

[John348]

I have used DOT 5 for over 25 years and never had a problem, I would never even consider going back to DOT 3. I just went through the system on my 60 Impala last year and after 20 years the fluid came out as clean as the day it went in! 

Edited June 24, 2016 by John348 (see edit history)

[Curti]

Quote

I also have DOT 5 in my '54 Metropolitan.  The only other problem I've had was the brake light switch.  They seem to fail periodically with the DOT 5.  

Yes, this is true with NOS brake light switches.   Buy a new one from NAPA, problem solved.  I would never go back to DOT3

[R Walling]

What is the Difference between DOT 4 and DOT 5.1 Brake Fluid?

 

DOT 4 and DOT 5.1 are both glycol-based brake fluids and are used widely in the automotive and cycle industry. They are controlled by standards set out by the Department of Transportation (DOT) - hence the name.

The main difference between these two brake fluids is in their boiling points. Part of the standards that need to be met by the manufacturers of DOT fluids are the minimum dry and wet boiling points. These are the minimum temperatures that the brake fluid must perform at before the brake fluid starts to boil, which can lead to complete brake failure.

Let's take a look at the minimum boiling temperatures of DOT brake fluid as specified by the Department of Transportation.

Remember, these are only the minimum standards. Brake fluid manufacturers can and often do improve on these figures and it is possible to find DOT 4 brake fluid with a higher boiling point than some DOT 5.1 fluids on the market.

Since DOT 4 and 5.1 are both glycol-based brake fluids they are compatible with each other, which means they can be readily mixed without harming your brake system. It is important never to mistake DOT 5.1 (glycol-based) with DOT 5 which is silicone-based and should never be mixed with any other DOT fluid.

So just which brake fluids can you mix without causing harm to your brake system? Let's take a look at the chart below.

Here you can see that silicone based DOT 5 is the odd one out and is not compatible with any other DOT brake fluid. By mixing DOT 3, 4 and 5.1 brake fluids, assuming it is fresh fluid, the worst thing that can happen is a drop in the boiling point of the whole fluid.

Some brake manufactuers, such as Hayes and Formula, pre-fill their brakes with DOT 4 brake fluid from the factory. Others including Avid and Hope, choose to use DOT 5.1 in their brakes. Many riders with DOT 4 in their brakes will opt to bleed with DOT 5.1 to benefit from the higher boiling point and improved heat resistance.

[DonMicheletti]

Boiling point is really not an issue with a vintage car. The real DOT 5 advantage is that it doesnt absorb water and for a car that isnt used much, the brake system is less trouble. With regular DOT 3 a brake system can fail, due to rusting by water absorbtion,  over time even if the car is never used.

I have had cars with DOT 5 in them for over 30 years with no problems at all. Unlikely with DOT 3 without maintenance.

 

But, as mentioned, some brake elastomers are not compatable and will swell, DOT 5 isnt for everything  (I found that out the hard way)

 

[Digger914]

Have used DOT 5 fluid in many old brakes and it oozed out from about a third of the older and not so previously well maintained systems. When it did, replacing the weak part with new stopped the leak. A friend of mine with a stable of old cars and lots of trailers that sometimes sit for years at time, hooked me up with DOT 5 for my boat trailer years ago and made me a believer. It works and it works well, but it doesn't always work the first time. You can flush the system squeaky clean, hone the cylinders till they shine like a new dime, put in all brand new seals and still ooze fluid. Replacing with a factory reman part doesn't always work and my experience with DOT 5 is that you need more than the right kind of seals, you need the right kind of tight and brake parts that are still in tolerance for the fluid they were originally made for aren't always tight enough to work with DOT 5.

 

I do strongly suggest against using DOT 5 in clutches that came from the factory with good old fashion brake fluid. Not that it won't work, but when it doesn't work, it's a lot of work to fix.

[Guest Skyking]

7 hours ago, Curti said:

Yes, this is true with NOS brake light switches.   Buy a new one from NAPA, problem solved.  I would never go back to DOT3

 

Yes,  since buying a new one,  it's been better.  The originals didn't last......

[Terrence]

Hi,

 

While searching the forums about my 1951 brake pressure switch not being compatible with silicone brake fluid I came upon this forum and now a week or so later would like to add my experience with Silicone fluid .

 

Here's a letter I wrote to Vincent Castillo of Cartel Products:

 

I first used Cartel 34 years ago in 1987 when I restored my 1951 Chevrolet aluminum step van and decided to try Silicone.  I had read the pamphlets on your product at Island Muffler in Victoria B.C. and decided to see if it was as good as the write up said.  the pamphlet said it was perfect for classic vehicles that can sit out of service for extended periods.

I finished restoring the truck and drove it around steadily for 15 years, then I retired it for twenty years out in the endless rain and some snow and just re-restored it in 2019. Believe it or not I have not touched the two front wheel brake cylinders since I honed them out in 1987! 

I'm using it daily again and the brakes are still functioning.

while I was RE-restoring it I decided I should put a new kit in the master and in the the two rear wheel cylinders even though they weren't leaking and still worked. To my amazement all I had to do was a little clean-up with fine steel wool to remove the slight stains where the rubber cups stop.  All the bores were perfect and needed no honing and a former service station owner was amazed as was I.  I took close up photos of the bores.

But now two years later my old hydraulic brake light switch has failed and i had have installed a different brand of silicone fluid.  "Blue Magic"  it is called . . . which I had a not-so-good feeling about from the start.

On your web site I see yours is the only silicone fluid that advertises, "compatible with synthetic and natural brake rubbers"

That old pressure switch of mine didn't fail using Cartel until 34 years later and I think it is because I changed the brand of Silicone fluid.  I should have gone searching for Cartel.  After two years years  WITHOUT Cartel the switch has failed . So I've come to the conclusion that not all Silicone Brake fluids are created equaland that Cartel is compatible with the sealing material in the old fluid brake switches!

My master cylinder is weeping as well even though the bore was perfect and I installed a new Raybestos master kit MK 33.  I'm worried about my wheel cylinder rubbers as well.

I want to purge my system and go back to CARTEL.

______________________________________________

 

So . . . Vince at Cartel sent me two 32 oz bottles of Cartel within 6 days and I converted the brake system back to Cartel.  Three days later, no more weeping from the master and the pedal, the pedal is rock hard and no more "blue Magic" dripping out when I pull the master dust cover back.

Interestingly, the "blue Magic" is a golden color but the Cartel is a nice purple/blue . . . which helped when pushing the old fluid out with the Cartel into my bleed bottle.  Two distinct colors remaining as two layers in the bottle with the Cartel staying on bottom of the two.  the Cartel also has 100 degrees higher boiling point and I suspect a heavier viscosity.  It wouldn't surprise me if my the "swelling' in the pressure switch subsided and allowed that to work again as well.  while I was waiting for the Cartel from Indiana I installed a lever type brake switch under the floorboards.  Like someone here said, no pedal pressure at all is required to light up the brake lights which is good.

 

Anyway that's my saga so if anyone is dealing with older style rubbers in brake kits . . . for myself after 37 years of Cartel in my 3.4 ton front wheel cylinders and never having touched those two cylinders . . . what more could a guy want.  Vince was very happy to hear that history.  By the way, Cartel is a Union Carbide company.

 

Here's a photo of the two fluids but the Cartel on the bottom doesn't look blue in the photo.

Other photo is my master without cleaning up the bore at all after 37 years and the kit that came out of it. Remember the truck sat out in the weather for 20 years as well.

 

Terrence

 

 

 

 

[Guest]

 I rebuilt cylinders 20 years ago and have not had to add one drop of fluid since.

[Paul Dobbin]

Restored a 1940 Ford Picup for the 1984 Great American Race, used Silicone Dot 5 fluid.   The Race cahnged the rules to 1936 & older

resulting in us not finishing the 40 Pickup until after the race.   Quickly prepped a 1934 Ford with mechanical brakes and ran the 1984 &

1987 races with it.   The 40 became a good driver for the next 20 years when it was sold to a Florida Museum Village for use as a service

truck to the Museum.   Gave them a bottle of Dot 5 Silicone fluid with the truck.  It's still being used today with that same Dot 5.

Did the same thing with my 81 El Camino in 2005.  It's still a regular driver today.  I'm going to Lowe's this afternoon with it.n

If you do the change pver, you need to get all the Dot 3 out of the lines, wheel cylinders and the master cylinder.  Once it;s all empty, flush it all with alcohol and air.  The use the Dot 5. and bleed thw whole system.   Don't forget to label the master cylinder so nobody adds Dot 3.   Also. some brake warning switches don't work properly with Dot 5.  9Just cover the warning light with black electrical tape.

Edited September 16, 2021 by Paul Dobbin
Added a thought (see edit history)

[Larry Schramm]

Blue Magic is Dot 3 brake fluid and silicone brake fluid is Dot 5.  Usually not compatible.

 

https://www.amazon.com/BLUE-MAGIC-INC-Brake-Fluid/dp/B07JZ4Q4PL

[jeff_a]

My 1928 Peerless manual says that its Lockheed Hydraulic Brakes should be filled with 50% Castor Oil and 50% Denatured Alcohol. 5th year for 4-wheel hydraulic brakes for Peerless.

[Terrence]

Larry . . .

 

The Blue Magic Silicone fluid  that i installed but have now flushed out in favor of Cartel, was DOT 5.  You had me worried and wondering how i could have mistaken a 3 for a five so I just went down to the shop to look at the Blue Magic bottle.  Here's a photo.  That Dot 3 Blue Magic you linked to wasn't silicone.

[Str8-8-Dave]

 

So I have used both castor/alcohol (DOT 3,4) and Dot 5 silicone in cars, trailers and motorcycles.  Dot 3 is fine for most applications short of competition where brakes get really hot, then Dot 4 is a better choice or if you are going GP racing silicone will definitely keep you off the boil as long as it is "fresh and clean".  Yes- the castor/alcohol base does attract moisture over time and will rust your steel brake lines and cast iron cylinder components or corrode aluminum parts like pistons if not maintained.  Boat trailers with surge hydraulic brakes are notorious for rust problems because brakes get dunked at the launch ramp.   

 

But in a different aspect castor/alcohol does better with moisture than silicone because the alcohol keeps the moisture suspended pretty evenly over the entire system, you don't wind up with condensed water droplets in low points in the system next to where all the heat comes into play, in the wheel cylinders or calipers where they are close to where kinetic energy is being converted to heat energy bringing your baby to a halt. 

 

"Fresh and clean" silicone based fluids WILL NOT absorb moisture but also WILL NOT prevent moisture from getting into the hydraulic system where instead of distributing the moisture evenly throughout the system it produces droplets of water that usually collect into slugs of water which wind up guess where- the hottest components in the system are, and will kill braking ability if it boils off as steam.  Castor/alcohol based fluid AND silicone based fluid in pure liquid state are not theoretically compressible with or without condensed droplets of water as long as fluid temps stay below boiling temperature.  Steam however, generated by slugs of water that finds it's way to a  wheel cylinder, is a gas and it is VERY compressible rendering a hydraulic brake system useless when the going gets hot. 

 

So, bottom line, both have unique problems with H2O, castor/alcohol keeps the moisture suspended but the alcohol, which bonds well with both castor oil and water, by nature, attracts moisture but then keeps the moisture suspended in the mixture.  It is a problem for rust and if it is really loaded with water degrades hydraulic performance.  Silicone doesn't absorb moisture like alcohol does but silicone doesn't keep moisture out of the system either and as water accumulates the droplets find their way to the hottest part of the system and when the water boils you have no brakes.   If you decide to change a system originally castor/alcohol fluid to Dot 5 silicone you also need to do a meticulous job of purging the castor/alcohol fluid because guess what else Dot 5 silicone fluid won't absorb or mix with- castor/alcohol solutions...

 

Either type of fluid therefore requires routine replacement, preferably on a regular basis, like after every race for a GP car running silicone, like every 2 years for us mortals who chauffer kids to soccer or on weekends an old warrior to a car cruise.  If you want to keep your expensive ABS system on your every day car working and prevent a warranty headache you should have the dealer change the brake fluid once every 2 years to prevent rust in the very expensive ABS pump and valve components which are particularly sensitive to rust and very likely run with castor/alcohol based fluids, not many OEM's recommend silicone, many will void any warranty on brake systems if they find it in your brake system.

 

Approaches I have thought of for prevention of rust in a system is use of stainless steel brake lines and, where possible, brake cylinder components.  That could be useful for alcohol/castor fluid if you are particularly opposed to just changing brake fluid every couple of years.  I have spent my time devising easier ways to change fluid rather than figuring out how to incorporate stainless items, using good brake bleeding equipment makes changing fluid pretty quick and easy and is an effective way to prevent rust.

 

PS:  I don't worry too much about all this brake fluid stuff for the old car, my 1931 Buick 8-66S has all mechanical brakes!

 

 

Edited October 24, 2021 by Str8-8-Dave (see edit history)

[Frank DuVal]

On 10/24/2021 at 11:43 AM, Str8-8-Dave said:

is use of stainless steel brake lines and, where possible,

Nicopp, Cunifer, both names of same material is great for brake tubing! Easy to bend and flare, but does not rust. Used by Volvo as OEM lines. You can bend them, cut, flare to fit while under the car. No need to buy them pre-made. BTW, Summit racing carries stainless steel flare nuts and stainless steel "armor" that spring looking guard on some OEM brake lines.

[Glen Andrews]

There's a lot of good technical information contained in this thread about brake fluid. I worked for Raybestos Brakes for over 22 years as a sales rep calling on warehouses, parts stores, and all types of repair/tire shops. Working for Raybestos afforded me a direct link to our product managers and technical service department at corporate headquarters, and access to products. Don't mix the glycol based 3 and 4 fluid with silicone, and silicone (dot 5) is really NOT recommended for anything with ABS brakes because it aerates very readily, and as mentioned in the above post have the dot 3 and or 4 brake fluid changed in your late model driver cars (with ABS) every so often. So moving on to putting it into an antique vehicle: it's best to have all new brake lines or clean out with denatured alcohol or a suitable cleaner, and let dry, and any part that has rubber parts need to be new (brake hoses, cups, seals.) Since silicone aerates easily do not shake the container and slowly pour the silicone fluid into the master cylinder, bench bleed the master cylinder slowly and patiently wait between strokes, and maybe let it sit overnight in the vise so any air bubbles work themselves out. Once mounted and hooked up then gravity bleed the system one wheel cylinder at a time; pedal bleed the system if needed, but once again don't be too aggressive with the pedal pumps and allow some time between those pumps. In 1991, after cleaning and replacing the rubber parts as I described above, I put silicone in my '76 Corvette, no problems until 2001 when the left rear caliper seeped, the calipers were stainless sleeved but the bottom of the piston has cast iron and had a little rust, so I cleaned it out and put a new caliper kit in, and all was good when I sold that car in 2008. In 1995 while doing the restoration I put silicone fluid into my '38 Chevy pickup and have not had a problem since, so that's 26 years and still going. The only problem with my '38 was the hydraulic brake light switch, I had to really exert force on the pedal when stopped in order for the brake light to come on, so I rigged up a mechanical switch which works fine. So obviously I've had good results with silicone fluid.