Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing DOT 5 Brake Fluid - Any Good?

Oh the two camp approach was alive and kicking way back in the '80's if that's when silicon first hit the shelves. Treat it like "For life" coolant, and in fact like EV's if I might be so bold.There are strong supporters who never faced problems, and there are also users who have had problems. So it's like anything else. Do your research,consider the risks, and then take a view.

I have converted several vehicles to DOT 5 when I completely redid the brake system. I have been happy with it, going over 10 years with no issues, The key is the system has to be cleaned of all DOT 3 or 4 (or 5.1) fluid. Just draining the system and refilling with DOT 5 will cause compatibility problems. I have not had to do any major brake work on my Plus 8, so I am still using DOT 4 and changing the fluid every two to three years. When I have to rebuild the brakes I will switch to DOT 5.

There's a little hidden bonus using old school DOT4, as every 2 years it makes sure you flush all the cr4p out of the rear cylinders and the bleeding process encourages a good detailed check of all the exposed bits like brake flexis

DOT 5 mineral fluid is used primarily for racing, it is more hygroscopic than DOT 4 therefore needs changing more often so ok in a race car. I think the jury is still out on synthetic fluid, ok I suppose in a brand new system. I stick to my DOT 4 mineral.

Sorry, but think you have that the wrong way round; DOT5 is LESS Hygroscopic.
One of its main advantages for racing is that it has a higher boiling point.

My Mog has been using Dot 5 since it was rebuilt by Kevin in the late nineties and during the entirety of my ownership since then, only changed once during the replacement of the torn dust covers on my front callipers, and while I was at it I cleaned up the rust on the exposed to the elements sections of the pistons, there was no need to change the fluid, as such but I thought to do so while I was also checking for seized bleed nipples all round....

According to that which I have read... Dot 5 was developed for the us military given changing brake fluid on every vehicle every two years for vehicles either in conflict zones or in long term, sometimes very long term storage, is a task that if it could be avoided, seemed worth investigating.... Thus DOT 5 appeared on the scene... and to the best of my limited knowledge... NEVER EVER to be mixed in a system that has had DOT 4 used in it....?

I suspect when Kevin rebuilt what is now my +8, he stripped and thoroughly cleaned any re-used brake parts fitted new seals or replaced cylinders in their entirety, new pipes and flexibles before filling the system with AUTOMEC DOT 5... and fitting a warning label to the master cylinder warning NOT to top up with anything other than DOT 5 .

I believe Automec Dot 5 is available at Halfords in the UK.

Please correct me if you feel I might have got anything even slightly incorrect or entirely wrong..

Hi all,

IMPORTANT

DOT 5 was developed for the US military. They wanted a fluid for their lighter vehicles that could better withstand extremes in temperature than the then available standard fluids DOT 3 & 4. Since then they have been trying to get Congress to allow them to switch back to a regular glycol-based brake fluid. However, safely swapping a brake system from silicone fluid to glycol base or vice versa is tedious, costly and dangerous. No automobile manufacturer delivers their cars with DOT5. Unfortunately, the USA Department of Transport US created a lot of confusion with the name. DOT5 is totally non-compatible with any other brake fluid I can recall. When combined with even trace amounts of a glycol brake fluid it can create, unknown to the user, a goo-like residue which can cause partial or complete brake failure. NO automobile manufacturer delivers their new vehicles with DOT5. Far too dangerous.

DISADVANTAGES of DOT5

a. DOT5 is not compatible with any other brake fluid or water. That fact can be very sad when combined with another more common type of fluid and most especially when the perpetrator is not aware. When combined with even trace amounts of other fluids, it forms a dangerous goo which prejudices braking, at times quite sadly. One must very carefully to flush the system if swapping from any other type of fluid with DOT5 or back. Other posts here note that.

b. One must accept that water will ALWAYS enter your brake system (regardless of what fluid you use) to a degree dictated by your environment. However, DOT5 is also incompatible with water. So the water that has entered the braking system "puddles" rather than combining with glycol brake fluid which will cause that fluid it to deteriorate but still not attack your brake system. These DOT5 water puddles internally attack brake components in the form of internal rust. However, like so much automobile wear, the effect is both invisible and so incremental that a normal driver doesn't notice. It can only be ascertained at a brake fluid change which the look of DOT5 discourages.

c. Of course, due to the factors above, DOT5 is immune to a quick brake fluid tester. If uselessly tested, they give a false, happy reading as the testers are deigned to determine brake fluids water content, which with DOT5 is always 0. DOT5 will also not change colours as glycol fluids do. So there is NO visIble evidence either of the water DOT5 is hiding. This even fools mechanics who only see crystal clear DOT5, regardless of much water is in the system, This even fools practised mechanics. I prefer the ease and certainty of testing. Testers are cheap and it only takes a second.
BTW,aside from becoming more vulnerable to heat and the rusting thing, water can be used as a brake fluid when in a pinch.Just get rid of it as soon as you can.

d. DOT 5 brake fluid becomes frothy when pressed on and off repeatedly. It results in the same effect (shiver) as ABS brakes.

e. DOT5 should be changed at least as often as any brake/clutch fluid, especially in the UK, where we kept a second Morgan for a wonderful decade. . Whether your car is used or not used, water enters the braking system. The safe brake/clutch interval change recommended by manufacturers is once every two years at least.

ADVANTAGES of DOT5

i. It will not invisibly melt car paint or/and human skin in minutes like glycol brake fluids so often do.

ii. It handles heat better than DOT3 or DOT4. Boiling brake fluid is no fun. However, it does not handle heat as well as DOT5.1 which IS compatible with DOT3&4, so keep read on.

CONCLUSION: I suggest DOT5 usage to only all Concours competitors like dear Kevin Vernon who Luddite mentions above. Kevin and I were close at one time. He restored our UK Morgan as he did Luddite's.

Secondly if you already have DOT5 in your car, I recommend that you keep using it WITH THE MAINTENANCE CODICIL. The risks and hassles of changing back are not worth it. GoMoG has a procedure to ascertain whether you have DOT5.

MY SOLUTION

If you want a better brake fluid, capable of handling higher heat (from exhaust or a potent engine) better than DOT3, DOT4 or DOT5 can move to DOT 5.1. DOT5.1, unlike DOT5, is an upgraded glycol fluid completely compatible with the first two (but not of course with DOT5.) Yes It is harder to find and a few quid more expensive but the compatibility means it will not cause any of the sadness noted above if you have to top up DOT3 or DOT4 roadside OR the other way around.

BTW, lately some upper class cars, like Volvos and Jaguar use DOT 5.1, along with ISO 4925 Class 6, in their newest vehicles. Ford also utilizes DOT5.1 in its performance cars. They call it Ford Genuine Fluid PM-21 DOT-5.1. DOT5.1 fluids also often contain rust inhibitors to prevent the corrosion of metallic brake components and to reduce oxidation at increased temperatures. It has a lower viscosity (aka thicker) than DOT 3 and 4, so it theoretically improves braking performance, especially in extreme conditions. However, I have have been using DOT5.1 for so long in all my Morgans my brake leg cannot remember the difference. Sorry.

And yes I am a Morgan trad nerd, with a curious memory for trivia.

Lorne

When my Harley was rebuilt in 1980 I used dot 5 as it was the latest trick and cool as a very cool thing .... later in the 80's I did a brake rebuild and went to dot 4 as it was a pest having one non-compatible vehicle in my herd of oddball wheeled-things ... can't say I noticed the difference in real life.

K

DOT 4 is really adequate enough and there is a higher temp option available should you need it. A new system or a fully refurbished system could arguably benefit from DOT 5 silicone but it costs and is hygroscopic and does need changing.
Rust in rear cylinders. Don't matter if you lube them or not any water absorbed into Dot 3 or 4 will rust the pistons. Oil as you know floats on water so the H2o element will stick to the metal surfaces and cause rusting. This also a-braids the rubber elements and you end up with that black sticky gunk which seizes the pistons solid. Now Dot 5 being silicone chases water out and it is the silicone that sits on the metal surfaces even in the presence of water which does tend to inhibit rusting and the system stays cleaner for longer.
And guess what, they are exactly the same in their action as an hydraulic agent unless you want to boil your brakes where of course silicon has a huge advantage but for a light road going car like the Morgan are you really going to do that. I drive my car quite hard on the rural welsh roads and I don't actually use the brakes much and even when you are they don't feel like I am using them to extremes.

I suspect the chance of boiling brake fluid may not be as rare as imagined on vehicles that spend some months or longer laid up, as many classic cars seem to..? ,Perhaps more likely in regions where the climate is such that it encourages corrosion and possible seizing of brake cylinders, or pads sticking to a mix of pins and or calliper castings causing partial or intermittent seizure, and dependant on severity can boil DOT 5 or other brake fluids to create a pedal to the floor situation when brakes are first applied..... After which pump the pedal rapidly in an attempt to slow things down..