I couldn't see an existing question about this but please link to it if you know of one ...

I should point out I'm English and the products described are bought in the UK, in case it makes a difference.

The car in question is a 1971 Morris Traveller.

I have always used DOT4 brake fluid with this, and another car of similar age, with no troubles.

However recently I bought another brand of DOT 4 brake fluid whcih had the word "Synthetic" written on it. Hmm. I don't think my usual supplier's bottles has this word.

After topping up with the new brand, a few days later the brake master cylinder gave way such that the pedal didn't operate any brakes (little/no resistance) but no fluid was leaking from amywhere. As a result I nearly had a nasty accident. Fortuately owing to a bit of luck the car and I escaped unharmed.

I see a few possibilities :

  • 1) The reason I had to top the Master Cylinder up in the 1st place was a hint at impending troubles, and the new brand of brake fluid has ntohing to do with this. or ..

  • 2) Mixing the DOT 4 and DOT 4 synthetic has caused some issue with the rubber seals.


  • 3) something I haven't thought of and won't know til I get it all apart.

Obviously I don't want to be mixing fluids wrongly but I also don't want to go replacing the brake system unnecesarily either !

So the question ..

Has anoyone used DOT4 synthetic & DOT4 'doesn't mention synthetic' at all ? Are they ok to mix together ?

thanks for all help

  • Never ever mix brake fluids for sealing reasons. Just use what's recommended by the VM. There are serious safety risks otherwise.
    – hornetbzz
    Nov 29, 2014 at 4:44
  • "sealing reasons" ? Can you explain a bit further ? Dec 2, 2014 at 9:32
  • Braking systems seals are to be compliant with the brake fluid DOT. That's why they are paired and it's not recommended to mix misc DOT for one braking systems.
    – hornetbzz
    Dec 12, 2014 at 10:36
  • @hornetbzz So ..to paraphrase .. you mean that because of the rubber-like seals in the car's breaking system, certain DOT rating is recommended (or insisted upon) by manufacturer - ie : Use the right DOT rating for the car ? Good point, I think that was missed in the answer as I had concentrated on mixing fluids of maybe same rating Dec 12, 2014 at 11:22
  • 1
    Not mentioned here, but DOT5 is NOT compatible with ABS, so you cannot use it in any modern car. It is used more for military vehicles.
    – Brain2000
    Sep 13, 2021 at 1:40

5 Answers 5


The information & links posted by Paulster2 offer excellent explanation of what the various types of brake fluid are.

There are a few grade :

  • DOT3
  • DOT4
  • DOT5
  • DOT5.1

The difference between them is a specificed standard regarding the boiling temperature, which rises as the DOT number rises, and amount of water absorption.

There are based on two materials: Glycol based ("traditional" brake fluid) and Silicone based. DOT5 is exclusively silicone based because only silicone meets the criteria specified by the DOT5 standard.

As regards mixing them: Generally this is a bad idea, but the main thing warned against is mixing Silicone based fluid with Glycol. This thranslates to not mixing DOT 5 with any other DOT standard fluid.

However mixing for example Glycol DOT3 with Glycol DOT4 would give unpredictable performance. Nowhere have I seen anything saying "this is ok". I suspect the reason is that a mixture of formulations may give an unproven boiling temperature.

One apparent red herring is the word "Synthetic" - Glycol based brake fluids may or may not claim to be "synthetic". This doesn't necessarily relate to whether the fluid is glycol based or silicon.

In a nutshell :

  • Don't mix the DOT ratings, unless you're adding a higher rating resulting in a raised boiling point
  • Don't mix glycol based and silicone based fluids (ie don't mix DOT 5 with any other rating)
  • Given that axiom, you can mix brands of brake fluid for the same rating/base material.
  • In practice, you can go up in DOT rating as long as it is the same base material. Mainly regarding your comment about mixing glycol based DOT 3 and glycol based DOT 4 is half true/misleading. You can mix the two and it will suffice for a system needing the DOT 3 rating but will boil and therefore have moisture/system issues in a system requiring DOT 4. Why does that matter? You can flush a DOT 3 system and fill it with DOT 5.1, the DOT standard is just a specification for characteristics of the hydraulic fluid. Sep 26, 2017 at 19:51
  • 1
    The glycol vs silicone is true because they don't actually mix, so you have two separate fluids acting in the system, which generally results in inconsistent hydraulic pressure as the fluids resist each other. It would be just like mixing engine oil with water. Sep 26, 2017 at 19:52
  • 1
    @finleyarcher I wondered whether that was the case. I have since heard of cases where the rubber s4als whcih 'prefer' one type disintigrate if they're used with the other. Eg on my morris traveller, it expects Glycol-based fluid. If I put silicone in (even if all new gear and fluid), the rubber seals might not like it. Feb 13, 2019 at 10:22

I found this description of synthetic based brake fluids. According to the page, all brake fluids are technically "synthetic" in that they are man made and do not contain a petroleum base.

"Synthetic" brake fluid, as we think of it, has a silicon base. Non-synthetic brake fluid (normal brake fluid) is glycol based. There are trade offs to each type. Silicon brake fluid does not absorb water, like glycol based fluid. It does however, absorb air, which makes it somewhat compressible. This gives the brakes a spongy feel to them.

Synthetic brake fluid should not be mixed with glycol based fluids. The article says you should wait until you need to do major maintenance (replacing large portions of your braking system) before you switch. When you switch from glycol, you should completely flush your system of the old fluid. It may take several "tries" to get it completely cleaned out of the former fluid before you attempt to utilize the vehicle.

Ultimately I don't know if the mixing caused the issues you experienced, but it seems to me it is plausible. Does it mean you need to completely change out your system? I don't think so at this point. I think if you went back to using glycol based brake fluid and completely re-bled your system, you'll probably be okay. I would do this before trying to purchase any new parts. Also, when you do change out the fluid, make sure you do some low speed testing of the vehicle in a controlled environment (such as an empty road or parking lot) prior to putting a lot of trust in the braking system again.

I also found this article which gives a lot of further information about brake fluids, especially where classic British motor cars are concerned.

EDIT: I did go back and reread the second article which is mentioned above. It reads thus:

If you do decide to convert to silicone fluid, it should be done as part of a total brake system overhaul, with freshly rebuilt or new calipers, wheel cylinders and master cylinder. Silicone fluid should not be added to a system that contains even small amounts of glycol fluid or contaminants. Merely bleeding the system is not enough, as there will be pockets of old fluid and sludge that will not bleed out. Silicone fluid tends to concentrate any residual glycol fluid, moisture and sludge into slugs instead of allowing their dispersal throughout the fluid, as does glycol fluid. This can lead to relatively severe but localized problems, rather than the more general system deterioration experienced with old moisture-laden glycol fluids. This may be a factor in reports of leakage when silicone fluid is used in non-rebuilt systems that had been operated with glycol fluid. A "new" system full of silicone fluid will require very little maintenance for years.

Again, don't mix the two. Only bad results can occur from this.

  • 1
    That's great information, thanks - from what I can tell, you shouldn't mix DOT 4 and DOT 5 as DOT 5 is silicone based, and DOT4 is glycol based. In fact the crux of it is don't mix glycol fluids with silicone one, but as DOT4 seems to be glycol and DOT5 is silicone, it amounts to not micing Dt4 and 5. But mixing DOT 4 with other DOT 4 seems to be ok. I guess it would have to be otherwise you're committed to one brand of DOT4. So it seems the problem was no. 1 : There was something wrong with my master cylinder anyway. Would you agree with this conclusion ? Nov 4, 2014 at 10:09
  • 1
    I read the articles, but came to a different conclusion. Synthetic DOT 4 is compatible with regular DOT 4. It's not recommended, but should not cause damage. What I would expect to happen is uneven water retention in the system (potentially leading to uneven braking). Nov 4, 2014 at 17:01
  • 1
    OK I think we have an opportunity to understand this fully, so let's do it. I think you have Synthetic equated to Silicone - I didn't read that anywhere (unless I missed it). Castrol GT LMA is quoted as being like "super DOT4" so I checked it out. Here's the MSDS: msdspds.castrol.com/ussds/amersdsf.nsf/0/… whcih confirms it's glycol based. DOT 5 is silicone based because only silicone products meet the standard. DOT4 could be, but so far as I can tell, isn't. I can't find any brand of silicone based DOT4. Nov 5, 2014 at 9:37
  • 2
    So my conclucions is: don't mix silicone with glycol. Some brake fluids put "synthetic" on the front but that's just advertising - they're still glycol based. Don't mix DOT 3 and DOT 4 and expect it to behave like DOT 4. DOT 5.1 is a high spec glycol based fluid, and should not be mixed with DOT 5, whcih is silicone. Nov 5, 2014 at 9:41
  • 1
    @user2808054 ... for the purposes of my answer, yes I was equating silicone with synthetic, which I understood was the basis of your question. There are synthetic/silicone based DOT4 brake fluids out there, so be aware. Nov 5, 2014 at 12:14

DOT 3, 4 AND 5.1 can be mixed. They are compatible. None of them are compatible with DOT 5 that's silicone-based.

I just bought some Motul DOT 5.1 and it says on the front label: "NON-SILICONE base DO NOT mix with DOT 5. DOT 3 and 4 are compatible."

The main difference between DOT 3, 4 and 5.1 are the boiling points, both wet and dry. The higher the DOT number, the higher the boiling points.

  • Thanks, that confirms what I thought - although mixing DOT 3, 4, 5.1 wouold result in a n unpredictable boiling point. But if you have say DOT 3 in your car, and top up with DOT 4, then you're just upping the boiling point so no worries. Feb 23, 2017 at 12:57

Manufacturers have been labeling their DOT3 and DOT4 products as "synthetic" as many consumers associate this with higher performance or quality similar to motor oil. But ALL brake fluids can be called synthetic. You can mix any of them except for DOT5. Even DOT5.1 is OK since this also a glycol based fluid.

If you mix DOT3 and DOT4 fluids (or different brands) you may get unpredictable boiling performance but otherwise it should not be a problem. Common sense says that if you mix two fluids, use the lower performing fluid's specifications if you are trying to predict performance.


All DOT3 & DOT4 fluid is synthetic...ie it is man made and does not have a petroleum base. DOT3 & 4 can safely be mixed. HOWEVER DOT5 (or 5.1) brake fluid is silicone based and must NEVER be mixed with DOT3 or 4.

  • I'd really be interested in your references on this? Can you flesh out your answer a little? Mar 5, 2016 at 15:05
  • there it is, in a nutshell Mar 7, 2016 at 10:13
  • 5
    I thought Dot 5.1 was glycol based, and only dot 5 was silicone, whereas this answer is suggesting otherwise. When it comes to brakes, I think specific facts are very important, as wrong info could be disastrous.
    – Michael F
    Apr 6, 2017 at 18:55
  • 1
    @MichaelF - Actually, looking at this, I couldn't agree more with you. DOT5.1 is compatible with DOT3 & 4, while DOT5 is not compatible with any of the others. Great point. Apr 6, 2017 at 20:42
  • 1
    The info in this answer regarding 5.1 being silicone based is inaccurate. Aug 1, 2019 at 4:53

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