Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Silicone vs. ordinary brake fluid

Recently there has been a thread about brake fade, possibly due to old brake fluid. Silicone fluid was mentioned, something to avoid. But why? I have heard many reasons, but what is really true, what is myth. I am not embarking on a crusade for or against silicone, but would really like to know what are proven facts and what is beliefs just passing from person to person citing each other.
I have heard:
Silicone will make rubber parts in the system swell
Silicone will make rubber turn into a goo
Silicone and ordinary fluid will form a gel if mixed.
Silicone is a bit more compressible and give a softer feel to the pedal
Silicone is routinely used i NATO vehicles (which are stored for long periods) without modifications, no issues.
Silicone can produce adverse effects in an old car with old rubber quality, but not so in a reasonably new car
True? False? Anybody know? And by that, I do mean know?

All this said, I do admit that I have silicone fluid in my car, but as I said no crusader - just curious. The car is from 2004, changed to silicone (after thorough flushing) about four years ago, no issues. The person who persuaded me has a total of seven cars, ranging from a modern Tesla to a pre-war Nash and MG, a Volvo PV 1800, a couple of old Merc and a late 90ies +8. No issues on any of them. I admit this is anecdotica
Are we on a highway to hell? Or have my friend and I actually seen the light?

Regards,
Robbie

Hi Robbie,

Let me steal from your post:

Silicone will make rubber parts in the system swell = YES..very possible
Silicone will make rubber turn into a goo = YES..very possible
Silicone and ordinary fluid will form a gel if mixed.= YES
Silicone is a bit more compressible and give a softer feel to the pedal =YES but only a bit that only racers or the very sensitive would notice.
Silicone is routinely used in NATO vehicles (which are stored for long periods) without modifications, no issues. YES & NO, read below
Silicone can produce adverse effects in an old car with old rubber quality, but not so in a reasonably new car INCORRECT

You can find everything you want in all the scientific language one could desire all this out merely with a web search, but in a nutshell...(though I have summarized on gomog in the brake section for mogger convenience.)

Silicone (DOT5) and Glycol brake fluids are not compatible. When mixed, even a bit, they exhibit the characteristic you list and can attack certain types of brake seals with sad effects. As checking brake seals for such sensitivity is a hassle for the average owner, the wisest course is simply not to switch from what the manufacturer used prior to delivery. However, if one has done a thorough flush, as you have done, it shouldn't be a problem, and time has confirmed that in your case. If you choose to switch back to Glycols, of which there are many, do the same thorough flush.

As for the other downside aspects of DOT5, I wouldn't worry about them. However, silicone brake fluid should be changed, in normal climates, every 18 months rather than 3 years. In England or Vancouver/Seattle, make that 12 months. You see silicone fluid is hydrophobic. Rather than absorbing water and going bad like glycol, it does NOT absorb water which pools in the brake system, which is worse and can cause internal corrosion. But if you were smart enough and capable of successful flushing as you did, this is not a problem for you.

The NATO thing is an American thing. BTW, DOT means US Department of Transport. Silicone was mandated by Congress to be used in US military vehicles which NATO uses a lot of. At the outset of DOT 5, it looked great! But after the full story was revealed, the military applied to Congress to get rid of it. IIRC, Congress was worried about the risks entailed of properly flushing the stuff out and the immense cost of switching all those vehicles. The US military is stuck with it so NATO is stuck with it.

However, DOT5 has two great features to it:

1. It is FAR less toxic than glycol fluids, (not that you should substitute it for Guinness) and DOES NOT remove paint or skin or worse For these reasons, it is a good choice for Concours competition players.

2. It forced the world to develop better glycol fluids, with more cold and heat tolerances. DOT 5.1 has higher tolerances than DOT5. There are even better glycol fluids than DOT5.1! Heat tolerance, as we can see from the recent thread, is of significance. The tolerances of all brake fluids start declining immediately and continue until the next fluid change. Here is aDOT 3 example.


No. He is merely a Concours player I imagine, who has gone through the effort of thorough flushes like yours to make his cars less prone to paint disasters. When I am smart and not lazy, I use a liquid resistant covering over the wing when I change my glycol fluid, just like the pros do. I am very careful not to drop any of the stuff on my skin. I have been burned. Doesn't happen with DOT 5.

In short, I do not recommend silicone because, for most of us, there is no longer any advantage to it and there is a risk is switching, either way.

gmg

P.S. My annoyance with DOT 5 is really an annoyance with the US Department of Transport. Calling DOT 5.1 gets it confused with the VERY different silicone DOT 5. Why didn't they call it DOT 6?!!!

My only experience with silicon brake fluid was on a tr3 ... A neightmare !
Each I release the brake or the clutch pedal it was making a jet in the fluid bottle encapsulating air bubbles and after 5 min there was so many air bubbles in the silicon that there was practically no brake anymore !
It take me hours to understand every morning brakes where ok and why after 5 min in a city circulation brake was practically ineffective.
Maybe it's due to the tr3 bottle or the tr3 brake and clutch master cylinder (both are identical in design) but I did not try again on other cars.
Dada

If the reason for the glycol change intervals is mainly the hydroscopic property, why change silicone even more often if it is not hydroscopic? What other reason is there?

As I wrote, by not being hydroscopic, doesn't mean moisture does not enter a silicone filled system. It merely means it is not absorbed, degrading the fluid in there.
Instead it stays and grows in the system, but divorced from the brake fluid. As it is in pure form, it begins to corrode the vulnerable metal inside the system, which is not an advantage.
Brake fluid is brake fluid as it does not erode metal.

Curiously Heinz, in an emergency, (and having no brake/clutch fluid in the car as Maurice Owen always did), you CAN fill the system with water only to get home. Water is not compressible. That being said, flush it out with brake fluid as soon as convenient.

gmg

I had a Harley which came with DOT 5 from the factory .... at the time it was touted as the 'wonder-fluid' .... wouldn't need changing ... just 'fit and forget' .... guess that idea didn't stand the test of time!! .... bit like fluffy concrete. 🙂

K

It's not Hydroscopic chaps

Hygroscopic = a substance tending to absorb moisture from the air.

Interesting thread on brake fluid. I am not a trained mechanic, thus anything I type can be taken with a pinch of salt..

Over a few decades of bringing old sports cars back to life, had I read the contents of this thread, high on my agenda of trying to bring them back to life would have been to replace the brake fluid.

When my old Mog was subjected to a nut and bolt rebuild at circa 8k miles, the fastidious chap who re-built it used Automec DOT 5 brake fluid, which was still in the Mog some four years later having enjoyed quite a few thousand miles of HOT weather driving on Southern European road adventures which my good lady and I greatly enjoyed in the Mog. That same fluid lived in my Mog during the many years it was mostly laid up while I was deep into a self build home extending programme... My guess is that all told that same brake fluid was in my Mog for perhaps 18 years, and caused no issues whatsoever, the car being driven only very rarely during my home extension self build process..

I did however change all the brake fluid a couple or so years back when I decided to overhaul and clean the front callipers having initially spotted a hole in the dust seal of one of the pistons, and having removed the pistons to polish the surface corrosion off them, during that process much of the fluid was lost, thus requiring replenishment.

When touring I ever carried a 1/2 litre of Automec.... for as has rightly been pointed out, mixing silicone brake fluid with the more usual and seemingly readily attainable non- silicone fluids, seems likely to cause major issues, which may require a full brake system overhaul...? Finding just the right brake fluid spec if you are unfortunate to suffer a brake issue when out on the road could be more of a problem if using DOT 5 as opposed to DOT 4 etc..?

I do not disagree with any of the advice given, just sharing my Morgan experience to date, and no I have no financial interest in Automec or Halfords where I believe DOY 5 was a stock item last time I looked..(-:

I suspect regular brake fluid changes might also help to avoid the seemingly one time common issue around seized brake nipples, in the process of regular brake fluid replacement they will all have been in regular use and assuming the fluid flushing programme has been carried out correctly...?

Just to add, none of the nipples on my Mog had seized despite the length of time the DOT 5 had remained in the system. Perhaps those Automec claims relating to laid up NATO vehicles might have some truth to them after all...?

Luddite, you missed something. That very talented & wonderful person was a Concours preparer, Tudor Motors, the best I ever ran across in the UK.
His work had John Worrall's 4/4 ranked second on Europe, ALL MARQUES. He prepared his/your gorgeous Plus 8 for himself originally and he
naturally used DOT 5, as Concours people probably should.

When he restored my UK Morgan, he knew I was a long distance mogger, not a Concours player. He filled the brakes, after refurbishing them with DOT 3/4.

Your post jogged my memory on an important point. As DOT 5 silicone is NOT hydroscopic but rather hydrophobic (where do they get these names??!!)
it will not react to a brake tester. and it will not turn the same colors as bad brake fluids of the regular kind. The water that enters the system will merely pool
in the wrong places and corrode metal like water normally does. It might go a smidgen yellowish or mauve. The manufacturers recommend a change every 18
months or less in a humid climate, whether the car is stored or driven. Some manufacturers say less 12 months, some more. Automex claims there fluid is good
forever. You to choose.

Regards to Sandra! Tell her we finally unpacked some stuff and I am now using her chrome espresso cup daily again!! High up in the Andes.

Lorne

My experience with Silicone -in an Alfasud with inboard front discs , when racing with the original fluid I always had brake pedal issues and wildly overheating front brakes, so we went to Silicone and all problems ceased -the Discs still glowed when i pitted but the pedal remained good -and the car stayed with that fluid for many years as it was developed with ever bigger engines. The only strange thing that I experienced was a solid pedal when the car had been parked in the hot sun for a day and prior to starting the engine. I believe the key to success was good seals and being scrupulous when flushing the system through.