Steering, Suspension, & Brakes

Hi, as anyone used silicone DOT 5 brake fluid for any length of time.
When I built my V8 2 years ago I used silicone fluid in both the clutch and brake systems. I started out with a dual brake system which had sticking problems, on and off, and after month replaced the master cylinder seals. The problem went away and of course returned a short while after....So I put in a single line brake system bled it and it has worked perfectly ever since. I had previously used silicone for 3/4 years with no problems in my previous B.

Last week my clutch slave cylinder leaked and I put in a new slave cylinder which lasted 3 weeks and 35 miles before it leaked. So I don't have a lot of confidence in the new seal kit working for long either.

Having read various bits and pieces it seems that the rubber seals can fail, but from my limited experience it depends on whoes seal kit you buy.

I have decided to replace all my clutch system seals, flush out the pipes with methylated spirits and refill with DOT 3,4 or 5.1.

Is this a reasonable action and more importantly should I do the same for the brakes as a leakage here can be a bit more disasterous.

I really don't want to do the brakes as putting in new caliper and wheel cylinder seals and replacing the rubber brakes hoses is a pain. So any thoughts will be greatly received.

Regards Ian

I only use Castrol LMA in my B. Works great.

I'm coming up on a year now with the DOT 5 fluid and have no issues. I have replaced all 3 master cylinders, not for leaks but to put in the correct diameters to make the systems work with correct travels and pedal pressures. A number of people have used DOT 5 for much longer and swear by it. One thing I did before putting the fluid in was to heat it on the stove to about 250 degrees to drive out any entrained air. There were quite a few bubbles that appeared and rose to the surface. When the bubbles stop, let it cool and it's ready to put in. Have a nice firm pedal now.

Good tip Jim.
Be aware that not all Silicon brake fluids are the same. Generally, you get what you pay for, so if you buy the $5 can it's likely to be something other than pure silicon oil. The $30 can is generally much better.

I've used slilcon in the brake system since 1980. I started out with part of a gallon jug of pure silicon oil with a bit of blue dye and it was great stuff. I was sorry to see the last of it and I still had that blue fluid in the clutch system until I last pulled the engine 5 years ago. In the brake system, I changed over to a cheaper, readily available purple fluid when the blue ran low in '89 and it was no better than the glycol-ether based fluids and deteriorated in just a few months which you could tell when the color changed. Eventually I found another high quality silicon fluid.

Most so called silicon fluids are a blend, containing some silicon oil and some other ingredients. "Synthetic" is a marketing buzz word and means nothing, as all of the ingredients are synthsized by some method. What it generally can be translated to is blended. Blended is not necessarily a good thing.

Some seal materials are attacked by some of these ingredients and matching seals to fluids can be troublesome. Blending in more ingredients can make it more troublesome. Silicon oil is an inert fluid and will not attack any known seal material. So, pure silicon oil, which is a pretty decent lubricant, is good for any seal type. The next time I need to buy brake fluid, I'm rather inclined to simply buy some pure silicon oil. I simply need to decide on the viscosity. BTW, mineral oil has also been used as brake fluid (Citroen LHM).

Incidentally, I have never had an issue with water in the brake system using silicon fluid. As mentioned, I kept essentially the same charge of fluid in the clutch system for close to 10 years between refills that were the result of slave type conversions where the fluid was drained but re-used the fluid the last time I did that as my supply of the original fluid was almost gone. Like I said, it was good stuff.

[en.wikipedia.org]

Jim

My 2 cents worth :-) I've use both the Dot 4 and Dot 5 w/o problems. I now use Dot 4 exclusivley. This type of fluid is compatible with the Dot 3 and seems to perform better and absorb little moisture. Acccording to various sources, the Dot 4 is all synthetic and meets the Castrol LMA specs. Valoline Syn Dot 4 is brand I've used recently. Only disadvantage to regular brake fluid is its paint removing capability. We do need to change it occasionally. It has performed well over the years and no one should be careless when installing it or bleeding the system. Each owner will have to evaluate their requirements and choose accordingly. Dot 5 does seem to swell the seals slightly. Since most of us are careful and use proven techniques, I see no reason to be concerned about the paint removing capability of DOT 3-4.

I've had silicone fluid in my car since about 1989. Haven't had any issues. I change it every few years and haven't had to rebuild the master or calipers/wheel cylinders during that time. I was running it in my clutch for a while but I went to a HTOB when I first did my conversion and they specifically recommended against using it so I changed the clutch to DOT 4. I have since gone back to an external slave and have continued to use DOT 4 in it. FYI.

Thank you for the replies.

The silicon I have used is from an American company Automec and is to spec. FMS116 and at $30 for 1/2 a litre it aint cheap.

It seems to have worked very well for two years and yes it is a pig to bleed and you can't use the stuff you bleed out for a while as it gets lots of air trapped in it. Didn't know I could heat it out.

My worry was that it seemed to be too much of a coincidence that the second slave failed so quickly and put it down to the silicone and not the after market slave cylinder.

Maybe I will see how the new seals get on and not fix what wasn't broke as far as the brakes are concerned.

I've had DOT 5 fluid in my Corvette since 1986. I've never bled it since the initial install and the pedal is still rock hard. I've also used it in every car I've built/restored since then and have never had a single brake related problem. One caution, never agitate the bottle. If you shake it, it must sit for a week or so. I've never heard of heating it but Jim's idea sounds like a good one. Also, when you pour it into the master cylinder, do it very gently so as to not aerate the fluid.

I think I read about heating the dot 5 on some brake manufacturer's website, wherever it was I guarantee it works. There is a lot of air liberated in the process. Keep an eye on the temperature so you don't overheat it, I kept in the 250 to 300 degree range. It went on for about 20 minutes before all the bubbles stopped appearing. Let it cool slowly and heed Jim D's advise about agitation so you don't put the air back into solution.

Silicone fluid is for museum cars. Cars which stand still a lot.

Cars which are driven or raced need a DOT 4 or 5.1 fluid.

Cheers
Chris

And just what do you base this on Chris?

Another mis-informed poster. I know of several racers that use it.

I guess I'll dip my toes into the silicone pond as well.
I've used silicone fluids for years and have been quite happy with them.
They aren't hygroscopic so no more internal corrosion of your expensive hydraulic parts.
And they tolerate more temperature before braking down.
The down side is the up front cost. (Which over the lifetime of the vehicle is actually lower due to the extended change intervals)
If moisture does get into the system (off roaders beware) it its catastrophic. As the two don't mix at all, it becomes very hard to remove. And starts corroding immediately.
Everyone has noticed the severe aeration issue. I get around that by back filling the system. Slowly filling the system through the bleeders from a pressurized vessel works like a charm. Bleeding by the "pedal" works as well but you must go very slow to avoid agitating the fluid.
The last word of caution is "do not mix silicone fluid with anything else". It does not play well with others! This means in particular, that if your system had any other fluid in it. Then you must flush all the lines and change all the rubber parts, including the flex lines. Rubber with absorbed dot 3 or 4 fluids in it will fail when exposed to silicone.
Some have claimed success by simply flushing and changing fluids, but that has never been my experience.
So for me, if the system is new. Then silicone is a wise choice.
If it has been used and not had every piece of rubber renewed then dot 4 would be the better choice.

Live like you mean it.
Fred

Looks like the consensus is that silicone should be OK especially as all the brake and clutch pipes, hoses seals were brand new when I built the car.

My worry was that I had had a problem originally with a twin circuit braking system locking on or not working at all. Which most probably was caused by the seals as I replaced new seals with new seals and still had the problem. Swapping for the single line brake set up cured the problem and it has worked perfectly for 2 years. Having said that it may have been a bleed problem but the brakes had worked initially for 2 or 3 months.

So when my clutch slave leaked I got a little worried especially as the new, after market, slave failed and then so did the new seals.

I suspect from what you all have said about bleeding and air bubbles that my problem may have been that when I bled the clutch I introduce thousands of tiny bubbles which initially let the clucth pedal feel OK, as I drove it and got an MOT the next day. The bubbles probably then collected and collectively they made the pedal very spongy. I did manage to pump the pedal and get home.

So now that I have bought all the seals and hoses I shall just cross my fingers and wait.

Nothing is ever simple but the problems do seem to be a lot simpler when the sun shines.



Edited 1 time(s). Last edit at 03/20/2015 10:27AM by hirot.

Ian, I didn't see if you use in-line break check valves? When we were building roadsters we used them in the beginning because the master cylinders were below the level of the breaks and about 3/4 of the cars had problems with breaks locking so we removed them. We used silicone fluid in all 22 roadsters without incident and a couple of the cars have been on the road for ten years, but as you say, every component in every car was brand new.


Paul.



Edited 1 time(s). Last edit at 03/21/2015 07:01PM by pspeaks.

Paul, didn't use in-line check valves just put in new copper pipes, joints, S/S sheathed hoses and used the standard MG brake and clutch Master Cylinders. The brakes are still working fine and I still have a very good clutch pedal.
The weird thing was when the clutch first went I just pumped it and got no pedal, went and told the wife the bad news, and when I returned there was pressure. It go me 70 miles to a 90th birthday and back home before the fluid had drained too low and drew in air.
When I put in the new clutch slave, 10 days ago, it worked the next day, then I left the car for 5 days and again had no pedal. Pumped it, waited, and drove 30 miles. However, it did leak fluid all over the floor.
Currently I have replaced the seals and have an excellent pressure....I am just waiting. Having now learnt about being carefull when using silicone I am hoping that I have made a better job of bleeding it .

I knew nothing about heating and not agitating silicone fluid, guess we were lucky, but now that I know I'll be more careful in the future. I'm using both MG master cylinders also but both, along with the calipers, were totally rebuilt and the wheel cylinders and all brake lines were replaced. Keeping my fingers crossed.


Paul

Its all sorted. It appears that the replacement slave had slightly different hose thread/length dimensions and I had over tightened the swivel nut on the end of the hose which had somehow flattened off the end of the hose which should have formed the seal at the bottom of the screw thread in the slave. So I had a small weep of fluid.
I have now replaced the hose and put on a new slave and very gently bled it and it works.
Thanks to all as I now know better how to bleed silicone fluid.