Brake fluid choice

McQ

McQ

Registered User
Hey guy's,
It's probably a stupid question to most of you, But my 78 Harley XLCH twin disc which has an aftermarket 11/16" master cylinder with braided lines running billet style four pot Brake Calipers... My question is which would be the better brake fluid to use, DOT 4 or DOT 3?
I am asking this as I read that DOT 3, although has a lower boiling point, Is less prone to go Hygroscopic (draw in water) Thus I'm thinking... "Well I'm hardly going to be pushing a lot of heat into those brakes, so I may be better off using DOT 3. What do you think?
 
Is there a reason you cant use dot5 silicon. far superior to my mind, except it dont work with ABS, higher temp tolerance, not hygroscopic, dont screw ya paint up. Harley used it as standard until the dawn of the ABS systems, I used it from Shovel to Twin cam with stock brakes and after market Performance Machine and Billet.
 
The issue with brake fluid being 'Hydroscopic' is generally down to basically laziness of maintenance and not regularly replacing the brake fluid!
Do not underestimate how hot your brakes will get, it's not necessarily related to how quick or effective the brake stopping power is!

A point to note is a 1978 model Sportster would have used DOT 5 as original fitment brake fluid.

The more important thing is not to mix or use silicon DOT5 in a system that has been using non-silicon fluid, i.e. DOT3 or 4 or vice a versa or trouble will follow!

That is not saying you can't but you would need to clean it and quite possibly replace all the seals to do so!
 
As an example I recently changed the DOT4 in my 2018 M8 softail slim, it's actually the first time its been changed since the bike was new, supposedly to be done every 2 years.

I measured the water content of the fluid I took out and it was less than 1% so just goes to show not really necessary to do it every 2 years as long as you monitor the fluid quality.
 
SJC69 said:
The issue with brake fluid being 'Hydroscopic' is generally down to basically laziness of maintenance and not regularly replacing the brake fluid!
Do not underestimate how hot your brakes will get, it's not necessarily related to how quick or effective the brake stopping power is!

A point to note is a 1978 model Sportster would have used DOT 5 as original fitment brake fluid.

The more important thing is not to mix or use silicon DOT5 in a system that has been using non-silicon fluid, i.e. DOT3 or 4 or vice a versa or trouble will follow!

That is not saying you can't but you would need to clean it and quite possibly replace all the seals to do so!
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What in 1978? Are you sure? I thought Dot 5 was quite new?
 
unclebubba61 said:
Is there a reason you cant use dot5 silicon. far superior to my mind, except it dont work with ABS, higher temp tolerance, not hygroscopic, dont screw ya paint up. Harley used it as standard until the dawn of the ABS systems, I used it from Shovel to Twin cam with stock brakes and after market Performance Machine and Billet.
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I didn't realise Dot 5 existed in 1978. So is Dot5 the best one to go for?
 
I also have a 78 Shovel and DOT 5 is definitely specified for that.

Extract from 1970-78 FL/FX Shovel Manual.

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Use whats specified and dont mix em as in the wrong combination it can lead to brake failure with so many categories dot 2,3,4,5,5.1 ect its not easy as harley specced different fluids for different models over the years i cant remember what does not mix with what 🤷🏻‍♂️
 
I'll post these links from a similar question in 2023.

Silicone brake fluid

Brake Fluid Ratings


What I would add here is when I have looked a few times at getting a so called 'Brake Fluid Tester' LED type pen, and again now, when I did my research, most actual 'technical' comparative testing said they are inconsistent, inaccurate and capable of failing even new unopened fluid with the only truly accurate & consistent testing done by boiling the fluid!

Bendix

"There are 2 common tools used to test brake fluid. One is a conductivity tester, the other is a boiling point tester.

Bendix does not recommend using a conductivity tester as they have been known to fail when testing freshly opened brake fluid. Conductivity testers estimate the moisture content by converting the conductivity into an estimated boiling point. Since brake fluids vary from formula to formula, this test is not an accurate result unless the tester is calibrated to the manufacturer’s product when new.

Bendix recommends a tester that heats the brake fluid to determine the correct boiling point. Using a boiling point tester is the only accurate way to test brake fluid performance."


Don't take my word for it do your own research!


I concluded, with ABS, it's just as easy to change it every two years regardless and don't bother with the tester!

I'll add a bit more,'if' the general manufacturer recommendation is to change Glycol brake fluid every two years due to being hydroscopic, you can be assured that this will be the absolute minimum to guarantee there would be no issues in 'general' use and the likelihood is it would be far longer than that before there would be any likely issues!

Read a lot of the component and testers manufacturers information and brake fluid testing can be seen as assisting in revenue generation.

Bendix again!


It takes less than a minute to measure the boiling point and confirm if the brake fluid has absorbed too much moisture to be safe. In the case on the following page, our wet boiling point is (155-200°C) which is above the Bendix DOT 4 minimum wet boiling point of 155°C. It’s important to remember that the system will continue to absorb moisture before the next service so it is recommended to change brake fluid regularly to ensure the vehicle is safe. By regularly checking your customers’ brake fluid condition is not only a vital safety issue but also a way of increasing your servicing revenue.
 
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You don't have original brake setup so Factory spec fluid is meaningless. I'd suggest you establish what is in system at the moment and stick with that, or do complete rebuild and flush system , then you can use whatever you like
 
Panhead Boris said:
You don't have original brake setup so Factory spec fluid is meaningless. I'd suggest you establish what is in system at the moment and stick with that, or do complete rebuild and flush system , then you can use whatever you like
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Yeah, I have done a full rebuild of the front hydraulic brake system, and just wanted to know what would be the best fluid... as the last time I did this it took nearly 3 day's of bleeding to get the brakes back! No leaks, Just couldn't clear the air lock, and had to leave it overnight with different remedies, and it eventually got there.. Thinking back it could have been contaminated by using different Dot grades... So what would be the best for easiness in bleeding? I have very high one off designed Fat Apes with a massive span..
 
The change from DOT5 was driven by issues with it and ABS pumps, it basically can froth causing bubbles which is not what you want in a hydraulic system. So the Tourers switched first as although ABS was not necessarily available on all models it was on the Police Bikes, that's when the switch started, 2003 DOT 5, 2004 Models DOT4, and the Sportsters later in 2007.

A good example is in the 2005 model year the Tourers were DOT4 but the Sportsters DOT5, so you do need to see what it says on the original master cylinder cap as to what was originally fitted!

Be aware if fitted with even H-D chrome accessory caps, around these times you needed to specify which DOT 4 or 5 to get basically the correct label on the cap, a lot of people just got one of the shelf and did not check if it was the right one!
 
SJC69 said:
The change from DOT5 was driven by issues with it and ABS pumps, it basically can froth causing bubbles which is not what you want in a hydraulic system. So the Tourers switched first as although ABS was not necessarily available on all models it was on the Police Bikes, that's when the switch started, 2003 DOT 5, 2004 Models DOT4, and the Sportsters later in 2007.

A good example is in the 2005 model year the Tourers were DOT4 but the Sportsters DOT5, so you do need to see what it says on the original master cylinder cap as to what was originally fitted!

Be aware if fitted with even H-D chrome accessory caps, around these times you needed to specify which DOT 4 or 5 to get basically the correct label on the cap, a lot of people just got one of the shelf and did not check if it was the right one!
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So you're saying Dot 4?
 
SJC69 posted some links earlier. Have a look at the one headed Silicone brake fluid. It covers both silicone and glycol, paragraph 3 is particularly useful.
With nonstandard master cylinder and brake calliper's it's going to be your decision based on the tech info and if you have ABS.
For me it's always Dot 5 silicone (does not eat your paint or need changing as often) as I have the first Harley anti lock twin disc brake system, i.e. nobody can squeeze that lever hard enough to lock the front brake. This was told to me by guy who had done a factory training course. Apparently the factory did not think that a system that would lock when grabbed in an emergency was a good idea.
Your trouble bleeding the system may be the layout rather than the fluid type.
The Brake Fluid Ratings is also a informative read covering why mixing different grades of glycol can be a bad thing and why putting Dot 5 (silicone) into a system that had glycol in it (any of the other Dots) is also bad. It also explains why Dot 4 can be better than Dot 5.1 depending on expected usage.
Good Luck.
 
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Ron said:
SJC69 posted some links earlier. Have a look at the one headed Silicone brake fluid. It covers both silicone and glycol, paragraph 3 is particularly useful.
With nonstandard master cylinder and brake calliper's it's going to be your decision based on the tech info and if you have ABS.
For me it's always Dot 5 silicone (does not eat your paint or need changing as often) as I have the first Harley anti lock twin disc brake system, i.e. nobody can squeeze that lever hard enough to lock the front brake. This was told to me by guy who had done a factory training course. Apparently the factory did not think that a system that would lock when grabbed in an emergency was a good idea.
Your trouble bleeding the system may be the layout rather than the fluid type.
The Brake Fluid Ratings is also a informative read covering why mixing different grades of glycol can be a bad thing and why putting Dot 5 (silicone) into a system that had glycol in it (any of the other Dots) is also bad. It also explains why Dot 4 can be better than Dot 5.1 depending on expected usage.
Good Luck.
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I'm going to go with the DOT 5, as I don't want my new paintwork spoilt! LOL :ROFLMAO:
 
Just my tuppence worth but......

You need to first establish what is currently in your brake system and stick with that. It doesn't really matter if you use DOT4 or DOT5, what matters is that you don't mix them. If you don't know what was in the system previously you will need to fully rebuild the entire system to ensure that you remove any trace of the old fluid.

Stu
 
Ratchet said:
Just my tuppence worth but......

You need to first establish what is currently in your brake system and stick with that. It doesn't really matter if you use DOT4 or DOT5, what matters is that you don't mix them. If you don't know what was in the system previously you will need to fully rebuild the entire system to ensure that you remove any trace of the old fluid.

Stu
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Yeah, As I say I have stripped the whole bike and am rebuilding and Customising. So I will put the DOT5 in. As I didn't realise it would take silicone Brake fluid in such an old model bike!
 
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