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MG MGB Technical - Brake/ clutch fluid

master cylinder archive about brake fluids, but I still have a question. Back in the 1960
GH Higginson

I can't believe you can still get DOT3. I have a 73 and a 75 and have been using DOT4 and Super DOT4 aka DOT5.1 (which despite the nomenclature indicating it is a development of DOT5 is nothing like it and it is not silicone-based) for nearly 30 years.

As long as you keep away from DOT5 silicone the other 3 are compatible with each other, but each successively higher number has better performance than the previous ones. Originally DOT5 was specified because of its higher boiling point, I believe LMA was similar, but DOT4.1 is equally as good. British cars hanging on to natural rubber seals as late as the MGB is a myth.

If a car has been sitting for 4 years with no fluid in the master than it's more than likely that had damaged the seals, and simple usage was enough to finish them off. Replace the seals, or if the bore is corroded replace it altogether, the original metal-can masters are available again - in the UK at least. If you replace the seals note that there are two kits. Suppliers show how you can identify from the outside which type were fitted ORIGINALLY, but one of the kits also includes the piston and the seals are different to the other kit, so you need to see what is inside your master first. More info here

Thanks Paul. I will take you advice and replace the master cylinder. I have replaced the other 2 components so why. Not the final one. Thanks again. MerryChristmas. George Higginson
GH Higginson

I echo Paul's comments. If the car was sitting for all that time, the fluid will have been steadily absorbing water and, hence, corroding the cylinders. You really need to examine ALL the cylinders, brake slaves and servo as well. If the car still has the original seals it's likely the part number is on the rubbers, but it sounds like you may need to replace the cylinders. They're your brakes, don't risk it!!
Allan Reeling


Here, stateside where George is, the DOT 3 is still common. Or, at least it was the last time I noticed a couple years ago.

C R Huff

In 2010 this Wikipedia page (I know ...) claimed most American cars built as of 2006 used DOT3, which amazed me. However that was in error as it was subsequently amended to DOT4. But I'm still amazed at the implication that cars produced up to that date may well have been supplied with DOT3.

The article does say "While a vehicle that uses DOT 3 may also use DOT 4 or 5.1 if the elastomers in the system accept the borate compounds that raise the boiling point" so another implication is that it was American-built cars that suffered seal degradation with DOT4, not British.

It wasn't that many years ago that you had to do a little shopping around to find DOT 4 here. And, when you did find it, there might be a shelf full of DOT 3 and just a few bottles of DOT 4. Or, you might find DOT 4 only in pints and not quarts.

Had time to make any progress, George?

C R Huff

I have not had a chance to work on my B. I hope to tomorrow. So I need Dot 4? When I rebuilt the brake system on my 1954 mgtf I used Dot 5 silicon fluid. Is that a problem?
GH Higginson

The trouble with a car that is new to you is knowing what the previous owner put in, and DOT5 silicone is a major problem especially if you want to change to something else. It is incompatible with any of the others and will form a sludge if mixed. It's a significant body of work to remove all traces by repeated flushing if you did want to go back to glycol-based.

It's only benefit is that is doesn't harm paintwork, after that it's all negatives. It doesn't mix with water, but the same amount of water gets into a brake/clutch system regardless of the fluid used. But in silicone it lies in lumps and can boil if it gets to the calipers, as well as causing localised corrosion. Silicone also absorbs air and can result in a spongy pedal, and it can also cause components to stick as it doesn't have the same lubrication capacity as glycol, which is why it isn't used in ABS systems with all the pumps and valves involved.

Unfortunately I don't know of a way of determining whether a fluid is silicone or glycol, apart from perhaps draining some off and adding water, as there are multiple colours for both. Water mixes very easily with glycol. I don't have any silicone to see what that does, but the implication is that it will lie in the bottom of your container and be visible.

If you are using glycol then any of 3, 4 or 5.1 will be fine, although if you intend using the brakes hard then the later the formulation the better.

I was surprised to learn that our 2005 built Mazda Tribute, purchased here new in 2006, used DOT3 brake fluid. I was advised by our local Mazda dealer about a year ago that they still use that in service for their cars it was specified for. I was also advised that I could substitute DOT4 for top ups if required.

Thanks Paul for the description of Super DOT4. I have been avoiding that concerned about the compatibility issue with DOT4.

R Taylor

My Rover 216 manual for 84 to 86 specifies DOT3. The Metro manual for 90 to 96 specifies DOT4, the Honda for 91 to 96 ditto, and the ZS for 99 on says DOT4 or Super DOT4. So cars for the UK seem to have changed to DOT4 by 1990 at least.

But the 1981 Haynes for the MGB also says DOT4!

I still have a bottle of DOT3 in my garage cabinet, I bought it after moving to Florida 4 years ago. All the braking components have been replaced and recently all the hydraulics for the clutch operations were replaced.

Now, what effect DOT3 has on any of the replacement parts, I do not know especially since I have had no issues with it.


79 MGB
gary hansen

The cars were designed to use DOT 3. DOT 4 has a higher boiling point initially but absorbs moisture more quickly than DOT 3. With either, moisture accumulation is an issue, especially if the cars are not driven regularly. The rust always seems to accumulate in the lowest part of the system.

I am using Castrol's DOT 4 with normal results in the B. As the midget was being stored for several years while I was living overseas, I converted it to silicon DOT 5. The pedal response is not as good (a bit squishy), but the problem of moisture accumulation has been solved. I have not had to replace any wheel cylinders or the master cylinder in over 10 years. I drive both cars regularly and do not object to the pedal feel in the Midget.

My conclusion is that DOT 5 is acceptable if the entire system (calipers, master cylinder, wheel cylinders and failure warning o-rings) is replaced, especially for a car that is stored or not driven regularly.

Glenn Mallory

The 'squishiness' is down to silicone fluid absorbing air, which glycol-based doesn't.

Can't speak for other master cylinders but on the MGB at least with its ventilated caps and a through passage for air as the system heats and cools, possibly containing moisture, as much moisture gets into a system with silicone fluid as with any other, even if it then behaves differently in silicone to other.

I can't see how moisture in glycol-based fluid, which will spread throughout, can only cause corrosion in wheel cylinders. The other source for moisture ingress is via microscopic pores in brake hoses, seals, joints and seams, so if wheel cylinders are corroding it's likely the ingress is there. Or maybe it's air ingress, as that is needed for corrosion or oxidisation.

The same ingress will occur with silicone fluids, but unlike glycol fluids moisture will pool at the lowest point i.e. calipers and wheel cylinders.

Not sure what effect usage has, any moisture that has entered a system can only be eliminated by vaporisation. And whilst the calipers may well get hot enough to do that the wheel cylinders probably won't in typical use. Even if it does in calipers, the vapour would have to migrate right back to the master and out of the cap, which unless the whole system is above 100C it won't.

That's not to say you can't experience what you have experienced, just that it may be for other reasons.

In all of these posts about brake fluid etc, no one has mentioned, the factory recommended service instructions, for the braking system, which is for UK cars, brake fluid change every 24,000 miles or every 18 months, all hydraulic seals replaced every 40,000 miles or every 3 years, and for North America cars the same with the exception on all hydraulic seals to be replaced every 36,000 miles or 3 years, instead of every 40,000, along with these instructions it also says all cylinder bores to be inspected for scoring, corrosion, and wear, and replaced as required, how many people reading this comply, I bet not many, I use DOT 4/ DOT 4+ in my car and replace the fluid every year on its annual service
Andy Tilney

In older cars using glycol based fluids, it is typical to observe rust pits at the bottom of the caliper pistons, at the low points in the master and at the lowest piston in wheel cylinders. I don't know why using the vehicle regularly seems to keep the moisture from concentrating and damaging the bores as severely as it seems to with cars that are parked for extended periods.

Regular brake fluid changes certainly help.

The silicon fluid is not hydroscopic but certainly any water that enters the system would not mix with the fluid and could damage the bores as well. I have not experienced this problem in the parked Midget while the brake system suffered significantly with the glycol based brake fluids. I don't advocate use of silicon DOT 5 but it has worked out well for the vehicle that is not in constant service.
Glenn Mallory

These days the seals in wheel and clutch slave cylinders at least are so poor that they end up getting changed, with the fluid, every couple of years anyway. I did change three out of my four masters at various points quite a few years ago now, but all with new-old stock, and before that they were almost certainly original. I had to change the roadster clutch master seals again a couple of years ago, I wonder how long they will last.

This thread was discussed between 18/12/2017 and 23/12/2017

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