Is DOT 5 as wonderful as some would have...

[geoberni]

An interesting article here: https://cartreatments.com/dot-5-brake-fluid/

Here's the salient points, just in case someone reads this in a few years and the article has gone:

Unlike other brake fluids, which are glycol-based, DOT 5 is silicone-based. The most prominent impact of this is that the silicone-based brake fluid doesn’t absorb any moisture. While this might sound like an inherent advantage of DOT 5, the truth isn’t that simple.

That’s because while DOT 3 and DOT 4 brake fluids absorb moisture, this prevents small amounts of water from building up and rusting brake components. As long as you don’t introduce large quantities of water into the system, everything runs fine.

With DOT 5, small amounts of moisture can ruin brake components and lead to soft brakes. However, if you’re servicing your brakes regularly, this isn’t as big of a problem. DOT 5 has minimal applications and is generally only used when you can’t prevent moisture from getting into the brake system.

DOT 5 is significantly more expensive than DOT 3 or even DOT 4. It’s typically three times the cost of DOT 3 and twice the cost of DOT 2. This leads to the erroneous belief that it must be superior.

However, the real upgrade for most brake systems is DOT 4 or DOT 5.1. While DOT 5 does offer some performance benefits compared to traditional brake fluids, DOT 5 brake fluid’s drawbacks outweigh the benefits for most applications.

Except in extremely rare circumstances, you should not switch to DOT 5 brake fluid. Furthermore, once you switch to DOT 5, you can never go back.

Even if you flush your brakes multiple times, you’ll never get all of the DOT 5 out of the system. It doesn’t matter if you ride a Harley Davidson or drive a Buick; once you switch, you’re stuck unless you replace all of your brake components.

Furthermore, you absolutely cannot switch to DOT 5 brake fluid if your vehicle has ABS brakes. Most newer vehicles have them, meaning that they don’t have DOT 5 brake fluid compatibility.

This is because DOT 5 brake fluid becomes frothy when pressed on and off repeatedly, which is precisely how ABS brakes work. Instead of stopping quickly, your brakes will become spongy, preventing you from stopping at all.

In addition to the added costs, DOT 5 brake fluid needs extra maintenance, or else you’ll end up with soft brakes and rusted components. If you’re thinking about making the switch, don’t. But if your vehicle already uses DOT 5, don’t switch back either.

So food for thought in the DOT 5 discussion.

[kevin s]

I have always thought as even now virtually no OEM's use DOT 5 there must be a reason, I stick to dot4 and flush a litre through every couple of years.

[les]

I’ve used it since my rebuild many years ago, there’s always two sides to a story. I suspect the reason for not being in general use, is the expense for a company.

[svenedin]

My car's braking system is in the process of being completely renewed. ESM recommended switching to Silicone and they have plenty of experience of it so having read up on the subject I have taken their advice.

[svenedin]

I would add to the above that the use of Silicone brake fluid is fairly common in the classic car community and it is also used in certain military vehicles. The reason for this is that it is not hygroscopic and so is more suited to vehicles that are likely to sit unused for significant periods of time. The military do not want to find that the brake cylinders are seized when they suddenly need to use a stored vehicle, nor do they want to devote resources to replacing brake fluid in stored vehicles. Morris Minors are pretty notorious for corroded brake cylinders and if you haven't had to deal with this I can assure you it is not fun. At the rear, the cylinders can corrode and jam in the on position if the handbrake has been left on. This makes it very difficult to remove the brake drum.

Of course there will be members of this forum who use their cars every day and those who use their cars on high days and holidays. Daily use cars and regular changing of the brake fluid should not have an issue with using glycol based fluid. It is really the occasionally used car that benefits most from a switch to Silicone.

I made the switch because the entire brake system was renewed; every pipe, union, cylinder etc. This debate has been going on for decades and decades!

Stephen

[geoberni]

I have always thought as even now virtually no OEM's use DOT 5 there must be a reason, I stick to dot4 and flush a litre through every couple of years.

Yes, not relevant to the use in Minors, but from this article it would appear that the reason for not using it in modern cars is it's unsuitable for use with ABS, which is in just about everything these days.
It's had it's day in that regard, DOT 5.1 is the 'new black' to borrow a phrase from popular culture....

[alanworland]

Filled the new system on mine 12+ years ago and not had to touch it or bleed it since, bit of a no brainer for an infrequently used vehicle I would have thought.

Alan

[King Kenny]

The rear cylinders on my Traveller used to seize up nearly every year. About twenty five years ago I replaced the rear cylinders, replaced the brake lines and fitted front disk brakes. For good measure i replaced the master cylinder. With all the new components I decided to use silicone brake fluid. It was not easily available back then. My local motor factors had never heard of it. Since then I have had no seized cylinders or any other brake related problems. The brakes feel firm and reliable with no noticeable difference in use. I have only replaced the fluid once in all that time.

[Keith 66]

I used to run a selection of moggy vans, A seized up brake cylinder front or rear was usual at every MOT & it was an annual chore replacing one or more cylinders & bleeding the brakes. I acquired & rebuilt my Moggy truck in 89 & put it back on the road in 1991, It had a complete new braking system from the master cylinder outwards. I filled it with Dot 5 silicone fluid, from that day onwards replacing cylinders was a thing of the past.
Much is made of the cost of Dot 5 fluid,, Really???? Have you bought a cylinder or set of them lately?
The truck served as everyday transport for 18 years as a working boatyard truck. In that time i replaced several sets of brake shoes but never had to replace a cylinder. I had to bleed the brakes just once when the hydraulic brake light switch failed & had to be replaced.
I laid the truck up & it sat for 7 years on the driveway. It was then driven into the workshop for its second rebuild (or is it third?)
After 7 years sitting still the brakes were still free & worked. The fluid in the master cylinder still the same clear mauve colour that was put in all those years ago.
How many cylinders would i have gotten through in all that time?
There is no way using old fashioned hygroscopic fluid is cost effective, its penny pinching but costs far more in the long run.
If you want your brakes to just work & keep on working year after year use Dot 5 its a no brainer.

[King Kenny]

Well said Kieth. Could not agree more. I have seen a lot of rubbish printed about Dot5. I cant praise it enough. It is a YES from me.

[kevin s]

Always divides opinion but the bottom line is both are fine if serviced properly, flush dot 4 through every couple of years and you won't have any problems either, my Elan has had no new brake components in 25 years.

Even with dot5 I would flush it every few years, it can still get water in the system (just doesent disolve).

I would put dot 5 in a brand new system but just keep to regular chages in a sytem which has been previously filled with dot4.

[simmitc]

As with others, I used to change one or two cylinders on each of three cars every year. Changed to DOT5 silicone 20 years ago and no more seized components. Also, if you do spill any, it does not strip paint. No contest.

[dp]

As with others, I used to change one or two cylinders on each of three cars every year. Changed to DOT5 silicone 20 years ago and no more seized components. Also, if you do spill any, it does not strip paint. No contest.

Yup, If you're not opening the master cylinder to change the fluid moisture isn't getting in to the system to do any damage. In contrast clearly the paint stripping normal brake fluid is doing something bad to slave cylinders.

[kevin s]

Most brake systems are not completely sealed. Every time you use the brakes the fluid in the master cylinder drops, this draws in a corresponding amount of ambient air (including whatever moisture there is in it) through the breather pin hole, this is why virtually every manufacturer recommends its changed every 2 years.

[geoberni]

Most brake systems are not completely sealed. Every time you use the brakes the fluid in the master cylinder drops, this draws in a corresponding amount of ambient air (including whatever moisture there is in it) through the breather pin hole, this is why virtually every manufacturer recommends its changed every 2 years.

Exactly.
No system can be completely sealed, unless it has some sort of membrane to allow the levels to drop in the MC.

[Keith 66]

If you enjoy changing expensive cylinders use ordinary stuff, if you want the brakes to just keep working use Dot 5 silicone, Its that simple really.

[Bill_qaz]

Most brake systems are not completely sealed. Every time you use the brakes the fluid in the master cylinder drops, this draws in a corresponding amount of ambient air (including whatever moisture there is in it) through the breather pin hole, this is why virtually every manufacturer recommends its changed every 2 years.

Exactly.
No system can be completely sealed, unless it has some sort of membrane to allow the levels to drop in the MC.

That's why motorcycle mastercylinder always have diaphragm allowing fluid movement without drawing in air/ moisture to fluid especially as rrsevoirs are exposed.