MGB Register Forum

Hi,
I know this is a basic query!

I have a suspected clutch fluid leak on my 67 b.
I was going to top it up to check the level drop but how do identify what is in there?

The fluid is clear.
The DOT 4 I have in the garage is yellow. So what does anyone think is in my car put there by the last garage?

Thanks in advance.

Ian

DOT4 should be practically clear, at least it will look that way in the clutch master cylinder.
It does turn more yellow as it ages- is the stuff in the garage new and unopened? If it's opened it looks like it is past it's best and shouldn't be used.

The only real incompatibility is if someone has put silicone fluid in there rather than a DOT fluid. I don't know what colour silicone fluid is, or how to distinguish it I'm afraid.

Ian F

I have silicone Dot 5 in my car and it is a light purple colour, although when it is very old it can become clear. It has almost no odour at all and is not flammable. I haven't got any glycol based fluid in my garage to sniff, but I think it has a chemical smell and it is VERY flammable.

Thanks Mike. I had a vague recollection that silicone fluid might be purple, but I haven't ever seen any
All DOT of course, DOT 5 is silicone, all the others are glycol.

Ian F

If the fluid hasn't been changed for years it could still be Dot 3. No problems with mixing Dot 3 and Dot 4 as they have the same glycol base. Top up with Dot 4 and bleed the old fluid out as it's time it was replaced

Just don't try mixing Dot 5 with anything as its quite different and doesn't mix with 3 or 4

I have read that you can use DOT5.1 with either. Although based on my experience with DOT5.1 on MTB brakes I wouldn't risk it.

I would assume that anyone keen on mixing Dot 4 with Dot5 may be addicted to brake fluid.

A pal of mine was a brake fluid addict but he used to claim he could stop any time he wanted

According to everything I have read you can't mix DOT 5 with DOT 5.1. DOT 5 is silicone, DOT 5.1 is glycol based just like DOT 3 and 4. All the glycol based fluids can be mixed.

Bumpa wrote: Thu Apr 10, 2025 4:05 pm According to everything I have read you can't mix DOT 5 with DOT 5.1. DOT 5 is silicone, DOT 5.1 is glycol based just like DOT 3 and 4. All the glycol based fluids can be mixed.

Exactly right. DOT 5.1 must never be confused with DOT 5 - why this nomenclature is anybodies guess.

DOT 5.1 is sold as a competition brake fluid, I believe it has a higher boiling point than DOT 4, but is more hygroscopic and should be changed more frequently.

Ian F