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MG TD TF 1500 - DOT 5.1 Caution

I went into the auto parts store today to pick up bottles of DOT 5 for my TD restoration. The counter person brings back bottles of DOT 5.1. Fortunately, I read the label. DOT 5.1 is NOT silicon-based brake fluid and is not compatible with DOT 5.

We have DOT 2, 3, 4 and 5. So why would they give the name 5.1 to a non-silicon brake fluid? Sounds like a major error waiting to happen.

So for those that run DOT 5, be careful when you purchase it. I would also not let a shop top up my fluid even if you are clear that you are running DOT 5.
Bruce Cunha

It was a stupid naming scheme. 5.1 is a glycol fluid, compatible with 3 and 4. They probably used 5x because of the heat rating. Same as DOT5.
Steve Simmons

Thanks for that warning Bruce. I agree the naming system is misleading. I run silicon fluid, so glad I saw this post before I made a mistake!

Cheers,
Bob
Bob Schapel

Whilst you don't want to mix things other posts on this forum have shown if it was to happen the consequences can be nil and certainly not the end of the world some tell us it would be. One person had unknowing a mix for years until he noticed a difference in colour on draining the system. The purple can loose it colour too forget exact reasons now.
Nigel Atkins

Mixing glycol and silicone won't end the world, but it introduces the possibility of malfunctioning brakes. It's a high consequence risk.
Steve Simmons

Agreed, Nigel and Steve. It didn't presented a problem in the case I know of, but I would not do it. The guy just bled silicon through the system in a Triumph 2000, without cleaning the system of glycol. It didn't seem to cause any issues.

When I converted my TCs to silicon (maybe over 40 years ago?) I couldn't resist doing some experiments. Unfortunately I did not write the results down but below are some results as I remember them. Check before assuming 100% correct!

1. Silicon floats on top of glycol. They would not mix.
2. The colour slowly leached from one to the other. (Think the silicon purple finished up in the glycol but not sure.)
3. Glycol would only mix with water or Methylated spirits.
4. Silicon would only mix with paint thinners or Turps? Kero', Petrol? (can't remember exactly).
5. Glycol on a painted surface doesn't soak far but lifts the paint, while silicon does not affect paint but seems to keep soaking along the surface forever.

I am not recommending this, but I have often wondered if a few millimetres of silicon floating on top of the reservoir in a glycol system would prevent the glycol from absorbing water from the atmosphere. The only other point of entry for water would be through the wheel cylinder seals, and that seems less likely. Perhaps I am overthinking!

Cheers,
Bob
Bob Schapel

Your list looks right to me. Interesting thoughts on a layer of silicone. Something feels wrong about it but hey, you never know. i suspect it wouldn't work because of the vibration and agitation, and then there's the mixing / goo issues outlined below.

When I switched my MGA to silicone I did some experimenting as well. Where silicone came into contact with contaminated glycol, it formed a white layer of goo. This goo is the real problem when it gets into orifices. The other issue is the different seal swelling characteristics of glycol and silicone fluids.
Steve Simmons

Cheers Bob.

Steve,
that was the thing as I remember the examples the (greater) possibility of malfunctioning brakes didn't appear present. I'm not sure now but I think someone also told me they swapped from glycol to silicone without more than normal flushing and without changing seals.

A mate said he put silicone in his B over 20 years ago and hasn't touch it since.

And I've seen where glycol has been in a system for very many years and all has been fine (the brakes are razor sharp).

I'm not suggesting anyone tries any of this, I haven't, I change my glycol brake fluid regularly and fully flush and bleed and my clutch fluid far too frequently and excessively flushed but that's another story).

But it does show that whilst it's normally best to be cautious some scare stories can be wide of the mark.

What contaminated your glycol?

Nigel Atkins

Normal contamination is water and rubber bits. Glycol fluid is hygroscopic so it will literally suck moisture out of the air. I've heard a few people say they just flushed with new fluid and never had a problem, but also many who tried it with poor result. Weeping seals, pulling brakes, disintegrating hoses, etc. Seems to be luck of the draw.

How well the fluid holds up also depends on other factors including how often it's flushed, how hard the car is driven, and most importantly what climate the car is kept in. In dry climates like the Western US deserts, you can probably go decades without glycol getting much moisture in it.

I have several cars with glycol and several more with silicone. They have all performed well and I don't have a strong preference for one over the other. BUT... in the MGA silicone has one massive advantage - when (not if) the master leaks all over the top shelf. Silicone can be cleaned up and does no damage, even a week after it happened. Glycol will immediately damage the paint, and in short order lift it right off and then promote rust on any bare metal it reaches. That's why I swapped my A to silicone. My two Bs and my C are also using it. My TCs all use glycol as does the Healey and Morris.
Steve Simmons

I ran the same silicone DOT 5 for at least 15 years without changing it. Probably not a great idea. I noted the purple color was lost, but the fluid was still a clear color. Not black that you see from long-term exposure of brake rubber to glycol-based brake fluids.


Bruce Cunha

Steve,
in the UK we often see moisture. 😄 I'm not dismissing any of your points and totally agree with the annoyance of glycol and paint, were in the 21st century if it wasn't so cheap and need changing at cost I'm sure a mass market alternative would have been found to glycol.

I think often those that get weeping seals, pulling brakes, disintegrating hoses, etc., is not because of the fluid but age and condition of those parts and how well and thoroughly the change was made.

And don't mention luck as if it wasn't for bad or very bad luck with cars I'd have no luck with them. 😒

Nigel Atkins

I have used Silicone Brake Fluid in our TF 1250 for seventeen(17) years. No issues in any way and the hose
connections still look brand new. Brilliant gear.

Our newly finished,fully rebuilt TF1500 has also been
using Dot 5.

However, both cars had new brake lines,new wheel cylinders,seals etc.fitted up during each cars rebuild.

Cheers
Rob Grantham
TF3719("Aramis"),TF9177("Athos").


Rob Grantham

The first year I awoke my TD after it's winter storage and the steel piston was stuck to the side of the aluminum slave cylinder due to moisture corrosion, I changed to DOT 5. It not damaging the paint is just a bonus for me.
Bruce Cunha

In August of 2013 I decided to totally replace my brake system, including the addition of an MGB brake servo. I had been anti-Si but I decided to try it. It's been almost 9 years now and the only fluid that I've had to add is a wee bit that I lose when I do a periodic system check by bypassing the servo (a simple technique with a piece of brake line that shunts out the servo). I still have the same small bottle of Dot 5 with which I finished the installation. Bud
Bud Krueger

This thread was discussed between 05/04/2022 and 07/04/2022

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