Can you mix dot 4 and dot 5.1?

[wemorgan]

Can you mix dot 4 and dot 5.1 brake fluid?

A Google search gives an inconclusive answer so does anyone know for sure what the answer is?

I guess people like to know why such questions are asked. Well, I'd like to change the brake fluid of one of my cars, of course. For other cars in the past few weeks I have used some dot 4 and some dot 5.1. So I have two very recently opened bottles. I don't want to throw either of them away for environmental reasons. So if it's safe to mix the two together I know I'll have enough for another fluid for one more change.

Thanks.

 

[JOB-BLACK-RAT]

Hi Will,

Brake fluid is hygroscopic (so it absorbs water, lowering its boiling point and so decreasing your braking). Once your bottle is open it is no longer sealed, dont store it just bin it.

Also you should not have to be topping up brake fluid so if you are having to do this you have a fault in your system and you must get this looked at asap.

Hope this helps.

Simon

EDIT: just re read that your chaning not topping up.

 

[Alex Crow]

Hi Will,

Brake fluid is hygroscopic (so it absorbs water, lowering its boiling point and so decreasing your braking). Once your bottle is open it is no longer sealed, dont store it just bin it.

Also you should not have to be topping up brake fluid so if you are having to do this you have a fault in your system and you must get this looked at asap.

Hope this helps.

Simon

EDIT: just re read that your chaning not topping up.

He did say both bottles were recently opened, and I am sure the have tightly fitted caps. The brake fluid in most cars has been in the system for years (not ideal!), so throwing away this new, capped, and recently opened fluid would be a waste.


Wemorgan, it is absolutely fine to mix DOT4 and DOT5.1

DOT5 is the one that cannot be mixed with other fluid types.

 

[Troon]

Edit: beaten to it...

 

[JOB-BLACK-RAT]

He did say both bottles were recently opened, and I am sure the have tightly fitted caps. The brake fluid in most cars has been in the system for years (not ideal!), so throwing away this new, capped, and recently opened fluid would be a waste.


Wemorgan, it is absolutely fine to mix DOT4 and DOT5.1

DOT5 is the one that cannot be mixed with other fluid types.

I bow to your greater knowlege

 

[Dieselman]

Brake fluid is hygroscopic (so it absorbs water, lowering its boiling point and so decreasing your braking).

Which then begs the question, why is it hygroscopic?

 

[wemorgan]

Thanks Alex.

I'm happy to accept that the net performance will be dot 4 rather than 5.1, but this is just for the commuter car so nothing fancy is needed. I just wanted it to be safe and not cause any damage to the brake system.

 

[simonegypt]

Dieselman, a small, general explanation-
Brake fluid is hygroscopic because it is made from glycol ether. Oil (petroleum) based fluids can damage seals, hence the use of glycol ether. The moisture is entrained throughout the brake fluid in very small droplets. This prevents pooling of water, which may possibly happen with a dot 5 silicon based product. One would want to prevent quantities of water forming owing to the low boiling point. Steam in a hydraulic line is to be avoided. I work extensively with gas and we use glycol systems for removing water from gas during production.

 

[markjay]

I understand that the issue for you is not financial but environmental (and at any rate Brake Fluid is very cheap).

But we always need to balance the two - running tyres until they are 1.6mm will be more environmentally friendly then binning the tyres at 3mm (and then get new ones manufactured for us).

So it's all about finding the right balance - and when it come to Brake Fluid I would always use good-quality fluid of the correct spec from freshly-opened containers. That's my view, anyway....

 

[WOODYTHEWISE]

In the days prior to most cars being fitted with disc brakes, ABS, better quality brake fluid, ensuring the fluid was changed on a regular basis was vital. Unless you regularly use mountainous roads I think it very unlikely that the fluid will boil if not changed for three years.
The reason I change/have the fluid changed (every two years) is any water in a system made of steel is a no no for me. Most brake pipes rust from the inside the same as exhausts. This may,or may not be required, but at the end of the day its my car.......
I also change/have the oil changed every three thousand miles, I know there is no reason to do this, however, each to is own.

 

[andy27168]

Hi,
I always bin part containers of brake fluid once I have used what I require.
I also won,t open a container of brake fluid till it has stood for at least 24 hours to eliminate any air that may of been introduced into the fluid during transit.

 

[wemorgan]

You're right the issue isn't one of cost, but more trying to minimise waste.

I wonder how quickly an opened but then resealed bottle of brake fluid absorbs water?
If it can be assumed that the brake fluid reservoir caps seal is as good as the bottles, then it could be argued the bottles have a 2 year shelf life (same as fluid change interval). Food for thought. But I'm talking about only a few weeks, not years.

No one has come forward to say mixing dot 4 and dot 5.1 will be detrimental to the car nor dangerous, so I'm going to give it a try.

 

[Dieselman]

Dot 4 and Dot 5.1 fluids are essentially the same being vegetable ester products, but Dot 5.1 has a higher boiling point.

 

[Dieselman]

Most brake pipes rust from the inside the same as exhausts.

Not on Mecedes cars, they definitely rust from the outside where the support clamps run.

 

[markjay]

But this is an issue for car maintenance in general... we are expected to renew gaskets, washers, bolts and even the fob batteries as a matter of course, not when they start leaking or failing. We are also told that we should replace shocks and springs in pairs, because if one dies the other will soon follow. But I worked in Africa, and nothing ever gets renewed there as a matter of course unless it actually dies a death, and then some...

 

[jefrs]

He did say both bottles were recently opened, and I am sure the have tightly fitted caps. The brake fluid in most cars has been in the system for years (not ideal!), so throwing away this new, capped, and recently opened fluid would be a waste.


Wemorgan, it is absolutely fine to mix DOT4 and DOT5.1

DOT5 is the one that cannot be mixed with other fluid types .

I may be a little dim, please explain that again slowly.

I can mix DOT 4 with DOT 5
But I cannot mix DOT 5 with DOT 4

That does not compute ?

 

[WOODYTHEWISE]

Its a mis quote.. dot 4.......dot 5.1...five point one.... not 5

 

[DRBC43AMG]

Check post #3. DOT 5 is a silicon based fluid and should NEVER be mixed with any others. It is true, that all these DOT numbers are rather confusing for those not too much in the know
Some reading here: http://www.afcoracing.com/tech_pages/fluid.shtml
or the usual Wiki: http://en.wikipedia.org/wiki/Brake_fluid

 

[WOODYTHEWISE]

Seen it.

Post 3.....OK to mix DOT 4 and 5.1 thats FOUR & FIVE POINT ONE

NOT ok to mix 5 with anything.

Post three is in agreement with you.