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MG MGB Technical - Brake Fluid

Hi All

I have replaced the complete brake system and I am now looking to refill.

I plan to use Dot 4 from Comma, as sold in Halfords, but before I do is there anything else worth considering, like Dot 5 or 5.1.


Tony Brough

There is a huge amount on this subject in the archives - notably concerning the MGA.
Roger Walker


I'm just about to fill mine with DOT4.

I don't like the sound of DOT5.

I used DOT5.1 in the race car, but not necessary for a road car IMHO.
Dave O'Neill 2

Thanks again Dave, Iíve added it to my lubricants and fluids shopping list!
Tony Brough

Definitely not DOT 5, it is not compatible with anything else as it is silicone based. The only benefit for you is that with a completely new brake system you could just pour it in, but then unless you do the same with your clutch system you would end up having to keep two different fluids. It's only advantage is that if it leaks it doesn't damage paint like the usual glycol types do, but after that it is all downhill. If you wanted to change from DOT5 for example, you would pretty-well have to replace the whole system again as all traces have to be removed before using a glycol fluid. It also aerates very easily giving a spongy pedal, and as much moisture gets into a system filled with DOT5 as with any other fluid, the difference is that it sits in one place and corrodes the parts.

DOT 5.1, also labelled Super DOT4, has the performance of DOT5 and is compatible with DOT4 if that is what is already in your clutch, but unless you are using the car for competition or racing down mountains DOT4 is almost certainly going to be perfectly suitable.

Definitely DOT4.

Then the question is whether Halfords offers the best value for money........I was given a Halfords voucher as a present last year and I haven't been able to find one thing in there that I can't buy more affordably somewhere else, other than complete Carrera bicycles (which are proprietary to them).
Peter Allen

I'm not going to say much except that I have had silicon fluid in my MGB for over ten years. A friend's immaculate MGB GTV8 used silicon fluid for over 17 years before he sold it. The fluid was never exchanged.

I don't have a spongy brake pedal and I have had to do absolutely no maintenance on the hydraulics since installing the fluid. When I replaced the front pads recently the pistons all retracted easily with no signs of them being stuck by corrosion. My MOT inspector, who is quite pernickerty, never mentions the brakes as being anything other than fine.

One thing is sure. Once you've made your choice there's no going back as the two fluids are not compatable. If I were building my car now I would probably use DOT 4 mainly because that's what garages expect to be in the cylinder, and as I enter my seventies I wonder how much longer I will be able to work on my own car. If it has to be serviced by a workshop it would be simpler with DOT 4. Don't forget that glycol based fluids should be exchanged every couple of years.
Mike Howlett

Both my 50 TD and my 67 BGT have dot 5 in them. For the TD, 15+ years without any change or leak. For the BGT about 9 years.

I am restoring my TD and just took the brake system apart. Clear fluid. Not a single pit or bit of rust in the slave and master cylinders. Rubber (or whatever it is made of). looks brand new.

If there is any disadvantage it is it takes a bit more to bleed off all the air.

I will put DOT 5 back in.

Bruce Cunha

Mike H, when testing a special in the IVA process it's a requirement to have a label close to the m.cyl with details of the fluid used. You might want to consider fitting something similar.

Best of...
PS, when you stop working on the car... bring it to me and you can direct operations!
M McAndrew

"PS, when you stop working on the car... bring it to me and you can direct operations!"

Ha, yes I'll be right over. What it really needs at the moment is a repaint. Ten years of use have taken their toll. Is that OK? See you in the morning :-)
Mike Howlett

HA HA, walked in to that one didn't I!!

line up your mates and get me two years of income at contract PM level and you're on ;-) might even give you mates rates :-)

best of...

M McAndrew

Just to add a footnote to the glycol v silicone debate. Read the "shaded" section.
Allan Reeling

Yes, I read the V8 register and I've seen that, but the cynic in me says that AP Lockheed would say that wouldn't they? They manufacture glycol based fluids.

I'm certainly not a hydraulics expert and I don't know the ins and outs of the debate. All I know is that in my own personal experience silicone fluid has given me no issues at all over ten years and travelling over 30,000 miles. My car's brakes are as effective today as they were when I first drove it in 2008. They don't "hang-on" and the pedal isn't spongy.
Mike Howlett

Never had a problem like stated in the article. Only soft pedal is usually when I first put in DOT 5 and need to bleed out more air.
Bruce Cunha

What follows is a good example of the sort of information that is floating around in the internet. Purporting to be from Automec:

So all DOT-rated products have to be compatible, but DOT5 and DOT glycol products must not be mixed!


I couldn't get your link to work, try this Brake Fluid.pdf

It would appear the link gets changed when submitted, it's because there's spaces in it, you'll need to copy and paste
R.A Davis

That doesn't work either, it's the spaces that seem to be causing the problem. Try:

Which works for me, even though it keeps adding the 's' to 'http'!

When I posted the link I hadn't realised the BB wouldn't be able to handle spaces in a link, I therefore edited my post to say you'd need to copy and paste. I suppose the moral is webmasters shouldn't use spaces in URL's
R.A Davis

This thread was discussed between 11/05/2018 and 16/05/2018

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