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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

 

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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 by Willie Wurke (see edit history)
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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.

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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.. 

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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

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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.

 

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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........

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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

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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

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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.

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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.

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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 by John348 (see edit history)
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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

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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.

Various boiling temperatures for DOT brake fluid

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.

Compatibility chart for DOT and Mineral Oil brake fluids

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.

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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)

 

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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.

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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......

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