Brake Fluids

[sjmmarsh]

I have just spent half an hour trawling the archives trying to find an answer to this...

 

I am going to bleed the brakes on my 7 as I have a soft pedal for the 1st inch of travel - it brakes fine after that.

 

I was going to stick Dot4 in it until the motor factor asked 'do you want mineral or synthetic'? I didn't know, so turned to BC.

 

The 7 manual just says DOT4.

The archives talk about mineral, synthetic and silicone...

 

What I can glean is:

 

1) Dot5 is silicone and is nasty stuff on a 7 as it can eat the seals on some brakes (esp AP 4pot brakes)

2) Dot4 and Dot5.1 should be interchangeable

3) Dot4 comes in mineral and synthetic

 

So my questions are:

 

a) Can I mix mineral and synthetic?

b) If not, how do I know what CC put in the car at the 24k service?

c) is silicone brake fluid mineral, synthetic or neither?

 

I have some Carlube Dot5.1 in the garage that says 'Do not use in Rolls, Jags or Citroens as these need mineral based fluid'. Does this mean that Dot5.1 is synthetic?

 

Steve

[SLR No.77]

Steve,

 

AFAIK DOT 3/4/5.1 are interchangeable, and are non silicone. DOT 5 is the silicone and is best avoided - seals etc.

 

I believe you can mix mineral & synthetic, but by the time the brakes have been thoroughly bled you'll have purged most of the system anyway. Brake fluid only has a short shelf life once opened (it's hydrophilic) therefore you may as well use the whole bottle

 

Silicone fluid is not classed as mineral or synthetic, but not relevent anyway because you won't be using it 😬 BTW, ditch the old bottle of DOT 5.1 you have and start afresh.

 

Stu.

[Peter Carmichael]

The DOT 4 and 5.1 standards include criteria for ability to mix with other DOT 3/4/5.1. Caterham won't have used anything other than fluids conforming to DOT 4 or 5.1. You can safely choose any DOT 4 or 5.1 replacement.

 

Don't get distracted by "synthetic/mineral" although DOT 5 is indeed not appropriate nor miscible with the other types of fluid.

[sjmmarsh]

Thanks for the replies guys. Clear, concise; just what I was looking for.

 

Steve

[Myles]

Be aware that Dot 5.1 is often referred to (verbally) as 'Dot 5' due to the rarity of true 'Dot 5' and laziness on the part of mechanics etc.

 

Millwoods got into a terrible confusion as to which they'd put in my car three years ago. Not an issue if you've got the bottle in front of you - but if you ask someone to change the fluid for you...

 

Project Scope-Creep is live...

 

Alcester Racing 7's Equipe - 🙆🏻™

 

Alcester-Racing-Sevens.com

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[anthony1956]

brilliant, just what I needed. 😬

[Sheds Moderator]

Citroen and Rolls use LHM, "liquide hydraulique minerale" which as you will guess from elementary French means that it's a mineral based hydraulic fluid. I think it is oil based, and it is dyed bright green to differentiate it. This is NOT EVER to be mixed with normal brake fluid, if you do the seals will rot and everything will go horribly wrong.

 

As Stu says normal DOT 3/4/5.1 are hygroscopic and draw moisture out of the air, hence the need to be dumped after 2 years. DOT 5 is highly hygroscopic and needs changing every few months, iirc.

[myothercarsa2cv]

LHM lasts forever 😬 (well, quite a long time anyway ) Bloody hard to find though!

[Roger Ford]

DOT 5 is highly hygroscopic and needs changing every few months, iirc.

No, it's hydrophobic - won't mix at all with water. Not that it matters because nobody uses it.

Brake fluid only has a short shelf life once opened

Surely only if you leave the bottle open? I can't see an issue with using brake fluid from a re-closed bottle. The amount of "wet" air in a half-empty bottle can't be significant, surely? I've never worried about using brake fluid which is a few years old. Should I?