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Brake Fluid: DOT 3/4 or DOT 5 (Silicone)?

September 24, 2010 | Troubleshooting & Repair

There's a lot of discussion about brake fluids. Should I use conventional DOT 3/4 or DOT 5 (silicone)?


Opinions vary widely on this topic. There's an excellent resource on brake fluids at the Vintage Triumph Website. The following text appears there:

"DOT5 does NOT mix with DOT3 or DOT4. Most reported problems with DOT5 are probably due to some degree of mixing with other fluid types. The best way to convert to DOT5 is to totally rebuild the hydraulic system."

Read the following posts from members of the 356Talk List and you be the judge.


Alan Klingen at The Stable wrote:

We have been very happy with ATE SL fluid, DOT 4. This is not a silicone fluid. It bleeds well, you will surprised how some fluids don't bleed well. It has been the factory fill till the last few years. This not the ATE Blue which is for racing. The problem with using a racing fluid is that they can lack the additives for long term use since they expect it to be changed all the time.


Bob Slayden wrote:

While at it I replaced fluid with silicone fluid. Blew out lines with compressor first then bled until all evidence of original fluid was gone. Many auto parts houses have DOT5 (silicone) fluid. I got mine at NAPA. Despite warnings about "spongy" feeling pedal, my car's brakes work better than ever. Good firm pedal with great stopping power. More confidence in the system. One of my other classics has had silicone fluid in it for 10 years. You can still see the outlet in the bottom of the reservoir through the clear purple fluid. Sure can't do that with regular DOT3!


Wes wrote:

I owned a Jaguar restoration and repair business up until 1986 and rebuilding the entire brake system on a vintage Jag could run into some serious bucks. . So when I did major brake overhauls on these vintage cars I would use Silicone brake fluid. And at least three of these people experienced complete breakdown of this fluid over time. The fluid became a gelatinous goo that was an incredible mess to remove from the system I have no idea what caused this to happen but suffice it to say that today I just flush brake systems every few years with fresh fluid and silicone doesn't come near my cars.


Ron LaDow wrote:

Aaah, I have some doubts...

If there's a chemist on the list, please chime in. My understanding is that silicone is inert (or pretty darn close). While I don't wanna cast asparagus, I'm tempted to believe there might be more to the story of the Jags' problems than silicone fluid.

I can tell you silicone fluid caused expansion of a particular square O-ring in an old Ferrari that was a bear to trouble-shoot (Dunlop parts; it locked the brakes), but caused no other problems in that car, or the Speedster (no problems at all). After twenty plus years, the fluid and the parts are pristine.


Linus Pauling Jr. wrote:

Quite a few of us have been using DOT5 brake fluid (silicone) for many years--in my case, 25 or so. I have found it a huge improvement with no, and I mean no, worries about corrosion, deterioration of hoses, damage to paint if spilled, etc. I have found no need to change brake fluid every two years as recommended with DOT3 and 4. In other works, it is a major technological advance. The only criticisms I'm aware of are that it is more difficult to bleed adequately and there may be some types of seals (although I have not experienced this) that react with the silicone. And, all the old brake fluid must be removed before adding DOT5 if the advantages are to be maximized.


Larry Dent wrote:

Silicone in old systems with old seals can cause failure. It happened to me on my personal 911 on rear disk seals. They swelled up and caused the back pads to wear out in 3000 miles. No effect on the front or the master cylinder, SO, my advice is standard fluid unless rebuilding the entire system, which is what you almost have to do if some cylinders are frozen, RIGHT!!!


Anonymous wrote:

I am embarrassed to recount the number of time I have switched back and forth between silicone and regular brake fluid. I did this about 4 times on my BMW motorcycle. Each time I heard a horror story I would switch. Currently I have regular brake fluid in all my vehicles and flush it every other year. Without any scientific study involving water retention in the brake system and rust. I have opted for the fluid that would retain the water without causing rust and flush it regularly to keep the boiling point high and the fluid clean. Clean brake fluid is like clean engine oil, it a must.

In theory if you sealed the vent in the brake fluid cap, no moisture would get in and the fluid would remain dry. I HAVE NOT DONE THIS. IN A PLEASURE DRIVING SITUATION I DO NOT BELIEVE THIS WILL CAUSE A PROBLEM BUT I HAVE NO RESEARCH TO SUPPORT OR DENY.

With my car which sits a long time. I have notice that the brake pedal must be pushed down hard to free the wheel cylinders (stainless steel) then after they are free I have no problems while driving the car. I cannot imagine why these would be sticking unless you did not lubricate them when reassembling them. Silicone brake fluid would work fine. I used silicon paste. My brakes are 10 years old and need to be redone again.


Jon Pegg wrote:

When I suggested converting to silicone brake fluid (DOT 5), someone on the List mentioned it may ruin the stop light switch. I went through my log for the car and noted that conversion was done (with master & wheel cylinders and thorough line flush in 1983). I've had no trouble with brakes since then, and the stop light switch (probably original) still works. Perhaps, like platinum plugs, this small sacrifice to originality is worthwhile.


Ron LaDow wrote:

Just checked the Speedster. It got silicone fluid, oh, about 1977. Brake light switch works just like the C coupe.


Linus Pauling Jr. wrote:

I know this subject has repeatedly appeared for discussion, but to me the advantages for our cars of silicone brake fluid are so obvious it needs emphasizing: Silicone brake fluid has been in my 4 disc-braked 356s for over 20 years. Operation perfect, no changing of brake fluid, no changing lines. One brake light switch has had to be replaced. These are all single-circuit master cylinders. However, on my '73 dual-circuit 911 I was never able to get rid of the soft-pedal syndrome with silicone fluid and went back to DOT 3/4.


Art Stanwood wrote:

When I converted from a single master cylinder to a dual, I sent to a silicone based fluid. My old brake switch immediately failed within two days. I contacted Klasse 356, from whome I purchased the system, and he suggested that the brake switch for the Harley Davidson motorcycle would do the job. I went to my local HD dealer and purchased the switch. Since all Harley's use the same switch, there was no decision about which one to use. I installed the switch and it has worked fine since. Of course, since the switch came in a plastic bag with the Harley logo on it, I had to pay a 30% preimum for it. It looked like a standard switch to me with a pipe thread fitting. Could have been off anything.

I suspect that a new original type brake switch would have produced the same results. Anyway, thats my story.


Phil Boughton wrote:

I also have been using silicone for 9 years in my 64C. I have not had one single problem. I took a cylinder apart a few years ago to see if it was full of gunk like the conventional DOT 4 and below produces if left in too long. It looked beautiful. I have bought several different brake bleeding tools and find that none of them work as well as having my boy push the pedal down and up on my command. I dislike bleeding brakes and will use silicone brake fluid in all of my cars until there is something even better

I think it is something like all the people who resisted using synthetic oil for so long (like the Porsche factory) then finally switched over after many of us had long realized the benefits.


Larry Dent wrote:

I don't know about the brake light switch, but I can personally attest to the fact that some of the rubber seals in some of the Porsche original factory built brake systems are NOT compatible with silicon fluid and will swell over a period of time and lock up the brakes.

I would suggest soaking any rubber seals in the fluid you intend to use before the re-build. If they swell, you know in advance what will happen in use. ---------- Woody wrote:

I'm a newcomer to the list and have some background with silicone.

My contacts at a high volume resto. shop confirm that EVEN THEY have had mixed results (i.e. one car great and the next problematic). As a chem. engineer, I've worked with the same fluid being used as a non-toxic coolant in freeze-drying process of biopharmaceuticals (< -40deg. C). Wonderful as the stuff is (temp.-stable, non-hygroscopic, lubricant), it still has the characteristic of foaming (holding air bubbles) much more than DOT 3 / DOT 4 brake fluids.

Brakelight switches are a good place for air to trap and would guess that's why some are going "bad". . . .

SO . . . if you wish to venture into the world of braking via silanes (silicone fluid), my belief is you will succeed or suffer at the hands of your own brake bleeding technique. Haven't tried it yet, but I'd avoid pumping the pedal!

I also recommend soaking replacement rubber seals in the fluid to check for swelling (measure bef. and after). Older or cheaper seals might fail as some budget silicone mfrs. may use low mol. wt. thinners to achieve viscosity target.


Doug Dutton wrote:

Silicone brake fluid is a great product for use in a complete new or thoroughly cleaned brake system. Once properly installed you should have trouble free use for years. You can get silicone fluid from NAPA auto parts stores and they are located all over. Hope this is helpful.


Dean Watts wrote:

I occasionally see people discuss Silicone brake fluid. Some swear it's the greatest thing since sex. However, be aware there are SOME rubber compounds that are not compatible with Silicone brake fluids. This is rare, but there are documented cases of rubber disintegrating and/or swelling in Silicone fluid. This could possibly cause a catastrophic failure in the brake system. An easy check is to watch your, (very clean new fluid, right?), and be very careful if you notice the fluid changing color. This could indicate a problem with a particular rubber component in your system deteriorating.

Again, I'M NOT CRITICIZING SILICONE BRAKE FLUID, but there could be a potential problem if you're unlucky enough to have a rubber component that may not be compatible with the fluid.


Al Zim wrote:

This [brake switch failure] seems to be a frequent happening with silicone brake fluid, the latest line of reasoning is that an air bubble gets trapped behind or in the switch. Start removing he old switch as you remove it have someone in the car gently put pressure on the brake pedal. This will fill the threads with fluid. Start installing the new switch as you thread the first thread, gently increase the pressure on the pedal so that fluid runs out. Tighten the switch and it's a done deal without bleeding the system.


Ken Daugherty wrote:

While Silicone brake fluid may be the best thing since sliced bread...I still would flush any brake system, or at least bleed the wheel cylinders a couple of 'pedal pumps' each annually. Just because...I am a bit anal when it comes to stopping power!

Many folks swear by silicone, especially for 'collector' cars. Regular brake fluid absorbs moisture. Silicone reportedly does not, but the moisture (water) can still be present in the form of droplets. I will use silicone brake fluid but still recommend that the wheel cylinders be flushed with a brake pedal stroke or two annually. This helps by removing any potential moisture and insures the bleeder taps are free. Also... it is good exercise for the owner.