Brake Fluid, DOT4, DOT5.1

[p.dath]

I decided to flush my brake fluid today (I was actually going to change the pads, but decided I had a couple more months of wear before the wear groves are gone).

Never bleed the brakes on the bike before. So a short session on You Tube and Google and I was under way.

First some stuff I didn't know (which may or may not be news to you). Glycol based brake fluids, aka DOT4 and DOT5.1 (which is probably what most of us are using) turn more and more brown as they absorb water. Once it turns brown it should be replaced.

My brake fluid was very brown. If your brake fluid is brown change it or get it changed.

Next I discovered DOT4 has been pretty much superseded by DOT5.1. And just for fun, there is also a silicon based DOT5 version. And for extra fun, if the two are mixed it forms a brown sludge in the brake system (bad).
As best as I can determine, if you mostly ride on the roads DOT5.1 is your friend.

Brakes feel ok now. But tomorrow is ART day, so they'll get a real good test then ...

[Jonno.]

Brown's when they're really dirty, I change before that.

Don't mix up 5 with 5.1..

[Coldrider]

DOT is a measure of its hydroscopy or something resistance to absorb water, don't get hung up on DOT 4 vs 5.1, as that is all it means.

[quickbuck]

The way I understood it, and somebody please correct me if I am wrong is this:

Dot 5.1 is LESS hygroscopic. ie, won't absorb water as fast , than dot 4.
HOWEVER Dot 4 has a higher boiling point. That is until you have old fluid full of water.....

SO, if you are running on the road with long service intervals, then dot 5.1 is better.
For Race applications, use Dot 4.

As a precaution, NEVER mix any brake fluid.

The fact that your pads take up the same space as the amount of fluid between FILL and LOW in your master cylinder is a good clue.
So, replace pads, replace fluid.

If you marely chuck in some more fluid and give them a bleed, then all you are doing is throwing good fluid on top of bad......

[nosebleed]

Oh dear. A hydroscope is a device used to look underwater.
The absorbtion of water by glycol based fluids is Hygroscopic.

DOT means Department Of Transportation, and has nothing to with absorbtion ability.
The number is the reference to the fluids boiling point characteristics.

The answer to all your Q's is MOTUL RBF600

[p.dath]

The way I understood it, and somebody please correct me if I am wrong is this:

Dot 5.1 is LESS hygroscopic. ie, won't absorb water as fast , than dot 4.
HOWEVER Dot 4 has a higher boiling point. That is until you have old fluid full of water.....

SO, if you are running on the road with long service intervals, then dot 5.1 is better.
For Race applications, use Dot 4.

From what I can read on Google, DOT5.1 is less hydroscopic as you say. What info I can find suggests it has a higher boiling point than DOT4. My impression from what I have read is DOT5.1 is better than DOT4 in almost all circumstances (except for maybe price?).


The fact that your pads take up the same space as the amount of fluid between FILL and LOW in your master cylinder is a good clue.
So, replace pads, replace fluid.

If you marely chuck in some more fluid and give them a bleed, then all you are doing is throwing good fluid on top of bad......

My fluid level was getting down close to low, so what your saying seems very reasonable.

When I say I gave it a bleed, I pretty much turned over all the brake fluid. I kept bleeding and fulling up the tank. Was I wasting my time? I was working on the theory this would predominantly leave me with new fluid throughout the system.

I decided against emptying the two tanks so I wouldn't have to worry about getting all the air out of the system.

[p.dath]

This Wikipedia article gives the wet and dry boiling points of the DOT standards. DOT5.1 is better than DOT5, DOT4, and DOT3.

http://en.wikipedia.org/wiki/DOT_4

[quickbuck]

From what I can read on Google, DOT5.1 is less hydroscopic as you say. What info I can find suggests it has a higher boiling point than DOT4. My impression from what I have read is DOT5.1 is better than DOT4 in almost all circumstances (except for maybe price?)..
You could be right.




My fluid level was getting down close to low, so what your saying seems very reasonable.

When I say I gave it a bleed, I pretty much turned over all the brake fluid. I kept bleeding and fulling up the tank. Was I wasting my time? I was working on the theory this would predominantly leave me with new fluid throughout the system.

I decided against emptying the two tanks so I wouldn't have to worry about getting all the air out of the system.
Well, i wouldn't say you were wasting your time.... You just did the tasks out of order.

No real hassle, as you will be well advised to open the bleed nipples when you push the pucks open in any case.
See, if you don't you will over fill the master cylinder, and then the pads will be stuck on as soon as any heat gets in them... without any leaver application.

[quickbuck]

This Wikipedia article gives the wet and dry boiling points of the DOT standards. DOT5.1 is better than DOT5, DOT4, and DOT3.

http://en.wikipedia.org/wiki/DOT_4

Okay,
I stand corrected......

I guess it is the PRICE then.

As the race bikes tend to have their fluid replaced more frequently, there is no point spending huge amounts on money on it.....

[Coldrider]

so in turn the brake fluid absorbs moisture which lowers the boiling point.

[CookMySock]

IIRC there is an incompatibility between those three brake fluids, and there is one combination that should be avoided. I don't recall what that combination was though.

Steve

[Owl]

This Wikipedia article gives the wet and dry boiling points of the DOT standards. DOT5.1 is better than DOT5, DOT4, and DOT3.[

I wouldn't get too carried away with what's better. As an example, Motul RBF600 DOT 4 way outperforms their DOT 5.1 in regards to boiling points, but lasts no where near as long.

I dumped the RBF after about 3 months and replaced it with cheap stuff from Repco. Certainly "better" for my application.

[Jonno.]

You could be right.

Well, i wouldn't say you were wasting your time.... You just did the tasks out of order.

No real hassle, as you will be well advised to open the bleed nipples when you push the pucks open in any case.
See, if you don't you will over fill the master cylinder, and then the pads will be stuck on as soon as any heat gets in them... without any leaver application.

What do you mean by this?

[quickbuck]

What do you mean by this?

You could be right: Well, he is partially right, as it transpires..... Some fluids are better than others due to manufacturer....

Well, i wouldn't say you were wasting your time.... You just did the tasks out of order: Replace pads and THEN change Fluid...

No real hassle, as you will be well advised to open the bleed nipples when you push the pucks open in any case: You open up the bleed nipples and allow the old fluid to flow out of the caliper. This means you are NOT pumping old fluid back up the system....

See, if you don't you will over fill the master cylinder, and then the pads will be stuck on as soon as any heat gets in them... without any leaver application: This is caused by there being no air gap in the Master cylinder for the fluid to expand.... Yes, it is a very small co-efficient of expansion, BUT it can happen because the pads are continually rubbing on the discs and the heat gets transferred to the fluid...

[Pussy]

A VERY good all round fluid is Castrol Response Super DOT4.
It's the stuff in the silver bottle. About $14.00 from SuperCheap.
The fluid is clear, and discolours as it goes off.
I don't use anything else

[imdying]

So, in summary:

- DOT 4 or 5.1 for your bike is fine, mixing during the fluid change process is fine too
- DOT 5 is only in some Harleys and tractors (a little ironic)
- The ratings you've read about aren't worth much, the DOT standard is a minimum, and most brands exceed that, some by a large margin
- Castrol and Motul brakes fluids are both of good quality
- How hygroscopic a fluid is isn't particularly important, if you're smart you'll change it every time you register your bike (i.e. every year) and that is sufficient
- It's sufficient to bleed the fluid through till nice clean looking fluid is coming out the hose (i.e. the same colour as you're putting in the top)
- All factory master cylinder reservoirs have a full mark, don't exceed that
- Some systems will make the fluid drastically discolour in days, just change it every year, or after a visit to your favourite track (or a coro loop if you're one of those types)

[Max Preload]

A VERY good all round fluid is Castrol Response Super DOT4.
It's the stuff in the silver bottle. About $14.00 from SuperCheap.
The fluid is clear, and discolours as it goes off.
I don't use anything else

Seconded. Castrol's Super DOT4 is good stuff and has better lever feel than DOT5.1. The worst for lever feel are the silicone based DOT5.


IIRC there is an incompatibility between those three brake fluids, and there is one combination that should be avoided. I don't recall what that combination was though.Just never mix DOT5 with anything else. The others are backward compatible.

[CookMySock]

Just never mix DOT5 with anything else. The others are backward compatible.Thanks for clearing that up. So DOT5 can be used in only new-fill or total rebuild + new hoses?

Steve

[imdying]

Yes, total rebuild with new hoses. Soak your seals in the DOT 5 for a day before installing them, just to check they don't swell.