View Full Version : silicone brake fluid

03-11-2003, 12:49 PM
Hi Folks,
The former owner of the BN-1 I just bought used silicone fluid in the car and the brakes are like stepping on a stack of wet sponges. I don't find yearly flushing of the fluid to be a big task and plan to change it to Castrol LMA or something similar. I know that change to silicone from a standard fluid can cause problems with leaking seals but am I ok to switch back to regular fluid with just a thorough flushing?

03-11-2003, 03:21 PM
You should have no trouble switching from silicone fluid to LMA. These two fluids will not mix. I believe the LMA is heavier and will reach the wheel cylinders just by breaking the bleed screws. Which will allow the silicone fluid to be forced out since the bleed screw is at the top of the cylinder. The same goes for the master cylinder. The silicone should be in the upper part of the reservoir. You could use a suction tube to pull the silicone fluid out.

Second thought, it might be easier just to open the cylinder bleed screws and drain the whole system. Refill with LMA and let that run out the bleed screws. Catch the fluid in a jar and check if both fluids are present. If so, repeat until only LMA shows in the jar.

03-11-2003, 03:57 PM
Hello 64lusso,
I have used silicone brake fluid for 20 years and would not use anything else on a road car. I have never had a soft pedal with it at all, are you sure there is not some air in the system, or some other problem, like weak flexibles?

good luck,

[ 03-11-2003: Message edited by: piman ]</p>

03-11-2003, 05:02 PM
The brakes on my BJ8 have been "siliconized" since the mid eighties and they are still very firm. I really need to refresh the fluid, I suppose, but they are still fine so I keep procrastinating on it. Bottom line is that my silicon break fluid system is very firm and I don't detect any appreciable fade in normal spirited (non-racing) driving. Suggest you check your system first before changing fluid - as mentioned a soft flex line will definitely cause this.

John Loftus
03-11-2003, 05:32 PM
I have read that you should avoid any mixing of the Dot 3 or 4 with the Dot 5(Silicone)or else you can get a gummy substance that will wreck havoc on your brake system. It is recommended to purge, disassemble and dry out the brake components before switching over. You might be able to get away with running a cleaner or solvent through the brakes but I would research it carefully ... I would worry about causing damage to the seals. Searching on Google should give you tons of info on this subject.

Cheers, John

[ 03-11-2003: Message edited by: John Loftus ]</p>

03-11-2003, 06:58 PM
I haven't tried to bleed the system yet and don't know what brand fluid is in it, hopefully that won't matter. My opinion of silicone was formed in the 80's when it first became popular a bunch of racer friends of mine tried it and got the poop scared out of themselves and all switched back almost immediately.
I tried it in another car I had restored in the early 90's and we had a heck of time with leaks in a freshly rebuilt system so I just didn't figure it was worth the effort but maybe I should buy a can and see if it will firm up a little with a good bleeding.
It feels just like a large stack of wet sponges when you step on it, a few pumps brings the pedal height up some but doesn't do much, if anything for the firmness.

Blonde Healey Girl
03-12-2003, 01:04 AM
Go back in the forum and do a search on brakes. There is some great info about proper bleeding. I bled all the wheels and the master cylinder the other day and it helped firm the pedal pressure. Have someone step on the brake pedal for about five minutes while you run around looking for leaks. Afterwards, you might want to get a bright light and get under the steering wheel where you can see the master cylinder and make sure nothing is slowly leaking down the firewall. If you don't know the history of the system, just replace the 3 flex hoses. They could very easily be original and they sometimes swell and or collapse internally. Keep that EBrake working, as it always helps to have plan B..........

03-12-2003, 05:06 AM
Hi 64,
Geoff Healey in his 1994 book recommended Castrol Girling brake fluid. He said there was generally no advantage to silicone fluid, except that it doesn't damage the paintwork. He does, however, specifically warn against the use of silicone fluid in "old brake systems" as these can affect the seals. Whether he meant old, as in old and worn, or old as in the older model cars, wasn't clear.

03-12-2003, 06:02 AM
Hello 64lusso,
further to Markb's comments (and there is a huge gain in not damaging paintwork anyway) the other gain is that the reduced chance of corrosion in the system, particularly on a car that stands a lot.
Also if you do plan to flush out the system, don't use methylated spirits as that will destroy the seals. I mention this because I have seen books that recommend Metylated spirtit as a brake component cleaner, which is fine but not for the seals.
good luck,
Alec graemlins/cheers.gif

03-12-2003, 12:54 PM
A friend of mine recommended using alcohol to flush out the system after removing the silicone before putting in Castrol, is that what methylated spirits are?
What I don't want to be doing right now is rebuilding the brake system because I know most of it was done during the restoration which wasn't that long ago milage wise, but can't say for sure it included the hoses although I think it did, so I think I will buy a can of silicone fluid and give it a good bleeding and see where that leaves me.
At some point I am sure I will do the switch back to regular fluid so this 'how to switch discussion is good'.

[ 03-12-2003: Message edited by: 64lusso ]</p>

03-14-2003, 02:51 PM
&lt;&lt;I know that change to silicone from a standard fluid can cause problems with leaking seals but am I ok to switch back to regular fluid with just a thorough flushing?&gt;&gt;

Hi Jay,

Keep in mind that when you do go to switch from one type of brake fluid to the other, they each use their own unique chemical formulation to cause the rubber seals to swell just enough to seal the system properly without leakage. After switching from one to the other, the seals begin to fail. This is part of (or most of) the reason people have had so much trouble with these brake systems over the years. If this is the case, even if you purge the system with alcohol, it won't be a remedy for this problem.

I have owned and driven various British cars for many years using both types of brake fluid, and neither have given me any trouble, except for DOT 3 or 4 eating the paint anywhere it dribbles. That's why when I restore a British car with a new brake system, I go with silicone.


03-14-2003, 05:56 PM
I just bought a can of silicone fluid a few minutes ago,(it's not too easy to find it turns out)and will bleed it this weekend and see where we are. Most everyone I asked at the parts stores agreed that the two fluids were not compatible and that the regular fluid should push the silcone out of the system if I decide to change, hopefully I won't need to.

03-15-2003, 04:46 AM
Hello 64lusso,
I don't rightly know if methylated spirits is alchohol, I do know it is mineral oil based. I was told by a Girling technical rep never to get it in contact with seals as it will destroy them. (There must be something similar to alchohol in it as some {desperate?} people do drink it)
I think that your idea of getting some silicone and trying to bleed them is worth a good go. If you are still not happy and decide to switch, I would use the new brake fluid to flush out the silicone.
good luck,


03-15-2003, 06:31 AM
Hi Everyone,
Its late And I just got home from work so Iam being lazy and not breaking out my Info book,so my facts might be off a little bit but should be pretty close. Silicone is used mostly in racing applications because of its higher boiling point properties, this is a plus.How ever a good race mechanic wiil flush and bleed his brakes before each race day.The down side of silicone is that it is very hydoscopic, if I got the word right it means that it absorbs water very fast. If I used the word wrong my vocabulary is bad but the silicone will still absorb water from wherever it can.It also does not mix well and ends up leaving small pockets of water in bends and hi spots in the tubing. This will cause the water to boil under Hard braking use making a gas and giving a soft pedal.This may have gone unoticed if the brakes are not used hard.This could also explain why it seems like it works good for daily use.All the articals I have ever read on brake systems stongly advise against using silicone for street use.I think I have most of the info correct.If silicone has been working for you no point in changing but you may want to consider these point to add to your evaluation of your brake system.
Hope this helps
( I wish I had a spell checker )

03-15-2003, 10:35 AM
Silicone fluid is Hygroscopic, which means that it does not absorb water. Any water that gets into the system will stay separated from the fluid, and will usually collect at the lowest point in the system. If the fluid (and collected water) isn't flushed regularly, this collection of water will corrode steel lines. The good news is that silicone fluid is also Hydrophilic, which means that it doesn't absorb water from the atmosphere. Any water that gets into the system will have to get there on it's own. That's typically not too difficult though, since there's a breather hole in everyone's reservoir lid. I also understand that silicone fluid tends to suspend air bubbles, making it harder to bleed all of the air out of the system (spongy pedal). As stated earlier, the up side is that silicone fluid isn't a paint remover. If you use it, change fluid regularly and bleed it very well. If you don't, use DOT 4 & don't spill any.

03-15-2003, 05:33 PM
Thanks Mick,
I had the facts backward but it at least got the right response.Next time I will try not to post from memory.Since silicone repels water it would make sense that it does not absorb it. Thats what I get for posting at 4 in the moring after a 10 hour work day.

Dave Russell
03-15-2003, 06:43 PM
Mick & CDK,

I think that you are both a little confused about terminology.

According to my dictionary, hygroscopic = afinity for water, hydrophylic = afinity for water, & hydrophobic = lack of afinity for water. Easy to get the phyls & phobes mixed up.

So silicone fluid is non-hygroscopic or hydrophobic.

Sometimes, the more simple words are easier to handle such as absorbs water or doesn't absorb water.

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by mvanderploeg:
Silicone fluid is Hygroscopic, which means that it does not absorb water. Any water that gets into the system will stay separated from the fluid, and will usually collect at the lowest point in the system. If the fluid (and collected water) isn't flushed regularly, this collection of water will corrode steel lines. The good news is that silicone fluid is also Hydrophilic, which means that it doesn't absorb water from the atmosphere. <hr></blockquote>

03-15-2003, 09:58 PM
I guess that would make me beeraphylic. graemlins/cheers.gif

Dave Russell
03-15-2003, 11:13 PM
Me too - definitely, but alas, wineophobic.

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by mvanderploeg:
I guess that would make me beeraphylic. graemlins/cheers.gif <hr></blockquote>

[ 03-15-2003: Message edited by: Dave Russell ]</p>

03-16-2003, 03:53 AM
Mick & Dave,
Thanks for making me think and keeping me honest.I do try to get my facts right before posting(helps in the validity department). Dave, always enjoy your posts as you always seem to back them up with research.It is starting to get warm and there is only about two feet of snow now, as soon as I can I want to get a look at my shock mount points and we can continue our past chat.In the mean time I will try to stick to monosylable words and ones I can spell.As soon as I can find the articals on brake fluid use I Will post valid low down.

03-16-2003, 11:05 AM

A simple way to determine whether you have silicone or glycol based brake fluid in an unknown system, is to place an ounce or two of fluid in a jar, and then add a drop of water. If the water droplet becomes absorbed in the fluid, then it's glycol based, if it doesn't mix, it's most likely silicone.


03-17-2003, 04:18 AM
Hi Jay

I agree with Mark. If it was good enough for Geoff it's good enough for me.

Cheers graemlins/savewave.gif graemlins/england.gif graemlins/savewave.gif

03-17-2003, 05:14 AM
Its late again but this time Iam awake and have facts
1.DOT 3 minimum boiling point 401 deg f
polyalklene glycol ether (hygroscopic)
2.DOT 4 minimum boiling point 446 deg f
polyalklene glycol ether (hygroscopic)
3.DOT 5 minimum boiling point 500 deg f
silicone (synthetic) (nonhygroscopic)
4.Dot number referes to the type of tests done
on the fluid
5.Hygroscopic-strong attraction to water
6.PGE based brake fluid absorbs water rapidly,
reducing the boiling point alot
7.boiling water causes gases which do not
compress well leading to a soft pedal
8.silicone and water remain seperated but water
is heavier so it settles to the lowest points
normally the calipers where it boils at 212degf
9.silicone fluids compressibility is unstable
thru temperture ranges causing a ever changing
pedal feel
10.brake part rubber can be ethylene propylene
which gets soft when exposed to silicone
11.do not use mineral based cleaners
12.DOT 5.1 not enough research yet
Hope this was better and cleared up some of the questions.For more information try www.thebrakeman.com/fluid_tech (https://www.thebrakeman.com/fluid_tech) I have read his articals for many years and they always seem to hold up.

RF Thom
03-21-2003, 12:52 PM
Valvoline recently introduced a Synthetic Brake fluid. This product seems to address all the concerns.
It's rated Dot 3 / Dot 4.
It will not absorb moisture - eliminates internal corrosion.
Will not attack rubber components.
AND it's more economical than silicone at $ 5.00 / litre in lieu of $30.00, and available at most auto parts stores [Autozone. PEP Bros, etc].

I put it in my Healey when I put it together, then I flushed out replaced the brake fluid in all the other vehicles [7] of my family which I also maintain.

Regards, Bob

03-21-2003, 07:56 PM
The Valvoline prouduct sounds interesting.

Having Put up a few posts on this brake fluid subject I realized that after a life time of working on brakes there was a lot more to learn.So alot of research and reading followed.Knowing how people feel when things have worked for them with no aparent problems I hesitated to add this post to a already long subject post list.However there was one outstanding caution in every artical. DO NOT USE SILICONE BRAKE FLUID.At best it was suggested for show or museum vehicals only due to its properties.From the previous post there seems to be alot of people using silicone.From a safety stand point my suggestion is do not. I am not a expert and do not expect any one to make changes based on my information.If you use silicone in a driver please take the time to do some research and then draw your own conclusions.It could make a difference.I have come to enjoy the people on the forum and have always loved Austin Healeys,lets do our best to keep both in good shape.