Mixing DOT 4 with DOT 4 synthetic

Question

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.

or

• 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

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.