Silicone Brake Fluid

[mrloboy]

I am in the process of finishing the brakes up on a '39 Plymouth Coupe project. The car is running an M2 front with 11" Malibu brakes, a 10 bolt Camaro diff with drums out back. A TCI Master Cylinder with 7" Booster is hanging on the firewall.  Willwood residual valves and proportioning valve are in place.

I am considering using Silicone Brake Fluid, mostly because of it's "paint finish friendly reputation". I have heard the rumors about spongey pedals etc., but have had no feedback from people that have actually had experience with the product.

Is their any performance benefits or deterrents? Your help would be greatly appreciated!  rj

[58 Yeoman]

From what I've heard, I'd stick with DOT4, and be careful when you use it around your paint. Just my opinion.

[buffalo bob]

I used Dot 5 (silicon fluid) in my '40 Ford for years with no problem with spongy pedal or any thing else for that matter.  One problem with the silicon is its ability to hold tiny air bubbles after the large air bubbles are cleared in the initial bleeding.  This would give the spongy pedal.  My friends that swear by the Dot 5, tell me it is very important to not shake the can prior to installing the fluid, thereby putting the hard to remove air in along with the fluid.  They like the higher boiling point of the silicon, the fact that it doesn't absorb moisture out of the air, along with not damaging paint.  I decided not to use Dot 5 in my F-100.  It is a pain to get the air out, it doesn't mix with Dot 3 or Dot 4, it is not redily available at local convenience stores (need to top off or emergency), and it cost more.  I'm just careful not to spill any when adding.

[Anonymous]

What Buffalo Bob said.  Used in wife's Model A and use it in my coupe.  No problems.

[purplepickup]

I'm not an expert, but I use DOT 5 fluid in my truck.  I used it because it doesn't eat paint but I've had a couple of problems and I'm not sure if they were related to the brake fluid or not.  

The biggest problem happened last summer at the top of Pike's Peak.  I didn't need the brakes on the way up but once I got to the top I drove up to the sign for a photo and the truck didn't have hardly any brakes.  I had to throw it in park to stop.....it's a long ways down and there wasn't a road in front of me.  I don't know what happened but on the way down I had minimal brakes until about halfway down.  At normal elevations I don't have any problems stopping.  In fact just after that I raced at the HAMB drags and easily stopped from 100 mph passes without doing any brake adjustments.

Other than that, the brakes are just a tad mushy.  Not bad, but not as firm as with DOT3.  

One other thing to consider is that even tho silicone fluid doesn't absorb water, that isn't necessarily a good thing.  You still can get water in your system from condensation and instead of it being absorbed by the fluid, it settles in low spots in your lines where it can cause rust thru.  My truck is in a heated garage and probably doesn't get much condensation but it's worth thinking about.  

One of these days I'll probably change back to regular brake fluid but when I do, I want to change all the lines and clean out all the silicone.  

To keep moisture from being a problem with DOT 3 you should flush your lines every couple of years and always use a freshly opened container of fluid whenever adding to the system.  Partially used containers get contaminated with moisture pretty quick.

[40cpe]

I've used the silicone fluid in two cars. I put it in my '57 Ranchero because I'm prone to let it set up for months or years without driving it and each time I would get ready to drive it the brakes would be leaking somewhere. I installed the silicone about 15 years ago and haven't had any more trouble. It has the master cylinder on the firewall and has a good hard pedal. Then I put it in my '40 with the master cylinder under the floor because of the success with the Ranchero. After trying all the tricks suggested by others I don't have a hard pedal. Like George's, it's a little mushy but at the top and stops OK. That's my experience, and if I were planning to drive the car every week or two I'd use the conventional stuff. By the way, I was in a parts store and saw Valvoline SynPower brand Dot 3-4 fluid that said it was synthetic. It said it mixes with regular Dot 3-4 and didn't mention that it was silicone. I don't know if it has an advantage over  standard dot 3 or not, but it just adds to the confusion.

[Charlie Chops 1940]

George,

I have read that silicone fluid is sensitive to atmospheric pressure changes - low pedal at higher altitude. When I put larger brakes on my '40 last year, knowing we were going to Pikes Peak, I switched to DOT 3. As it turned out I didn't take that car. However, your tale seems to confirm what I feared might be a problem. I have heard that Corvettes now come with DOT 5 - I wonder what the deal is there?

When I flushed out the DOT 5 that had been in the system  for 12 or 13 years I found no water & no evidence of rust. The pedal was always firm and it didn't seem to change any with the DOT 3 fluid.

My '61 Chev with DOT 5 always had a soft pedal, that I just learned to live with.

Charlie

[Bob Paulin]

The fact that it does *not* absorb moisture is, in itself, the problem with using silicone fluid in a daily driver.

Dot 3 & 4 fluids absorb moisture and hold it suspension.

Any moisture that gets into a system with silicone fluid, heads right to the lowest point of the system - usually the calipers and cylinders - where it simply sits and corrodes.

Brake fluid manufacturers - such as NAPA - send out bulletins stating that silicone should only be used in all-out racing applications and/or museum vehicles which sit for long periods of time.

I did an entire customer newsletter on this very subject, based on vendor information,  when I managed a NAPA store.

If I can find it, I will forward it to Fat Cat if he would like to stick it in the tech forum.

Silicone will also cause problems with anti-lock brake systems because of the creation and retention of air bubbles whenever the ABS cycles.

Bob Paulin

[HotRodLadyCrusr]

The biggest problem happened last summer at the top of Pike's Peak.  I didn't need the brakes on the way up but once I got to the top I drove up to the sign for a photo and the truck didn't have hardly any brakes.  I had to throw it in park to stop.....it's a long ways down and there wasn't a road in front of me.

Did you have to change your shorts after this???  

What a great thread.  Lots of info to consider.  One of the many reasons I LOVE this place.

[purplepickup]

Did you have to change your shorts after this???  

Prolly more info than you want, but if you really need to know the details, my butt puckered up so quick I didn't have time to mess my pants.    

[SKR8PN]

Did you have to change your shorts after this???  

Prolly more info than you want, but if you really need to know the details, my butt puckered up so quick I didn't have time to mess my pants.    

SOOOOOO George, how did you get the "pucker" mark,out of the upholstery?    

[Fat Cat]

If I can find it, I will forward it to Fat Cat if he would like to stick it in the tech forum.

Bob Paulin

And I have already added it here

[mrloboy]

Thanks to all of you for the replies. Your experiences and the knowledge of the subject is more than one could hope for.

Thanks to Bob and Fat Cat for the tech article regarding brake fluid.

I expect I will stick with DOT3 fluid in the car. It will be a daily driver and the customer does live in and around the Rocky Moubtains. Wouldn't want another episode like Georges "Pikes Peak Experience"!

[Dirk35]

Scroll down to the bottom of the page to get to the begining page. Its a good read.

http://www.hotrodheaven.com/tech/brakes/brakes2_index.htm
(WilWoods Brake Tech)

[Bruce Dorsi]

I have wondered why a light oil or hydraulic fluid could not be used in a brake system.

Is it due to a too-low boiling point or a flammability issue?

Is it prone to aeration, as is silicone brake fluid?

Years ago, oil may have had an adverse effect on the rubber compounds used in hoses, seals, etc.  ..... Today's synthetic compounds (Buna, Viton, Teflon, etc) would not be affected by oil.

What temperature is brake fluid generally heated to on a street-driven vehicle?   ....Oil in engines or hydraulics is often in the 200F degree range.  

I know I must be overlooking an obvious answer, but what is it?