Brake fluid: DOT 4 or 5.1

[Scud]

Based on some recent threads about rear brake fade, I got wondering about brake fluid. I've understood that DOT 3 and 4 are similar, but that DOT 4 has a higher boiling point, and I knew not to mix the DOT 5 (silicone) with it.

 

But I never understood what the DOT 5.1 fluid was for. According to this Wikipedia page, 5.1 can be used to replace DOT 4 and it has a 30-degree Celsius (45F) higher boiling point.

 

https://en.wikipedia.org/wiki/Brake_fluid

 

Obviously, water-saturated brake fluid boils more easily (and the data in the table at the above link are compelling). "Wet" DOT 4 fluid boiling point is about 80-degrees Celsius lower than non-saturated fluid.

 

Also obviously, brake fluid should be flushed periodically.

 

But not so obvious to me:  Is there any reason why we should not run DOT 5.1 in our bikes?

 

Has anybody used DOT 5.1 in a V11?

[thumper]

It is about three times the cost. I use it in my track bike, because of the high boiling point works well. Still must flush frequently.

If the rear brakes are fading something is causing it, dragging caliper, resting foot on brake lever, not flushing often enough.

I think it is more important to find the problem, a higher boiling point fluid may just mask the problem and cause issues with

the rear brake system. 5.1 Motul is good stuff and does work well.

[GuzziMoto]

You certainly can use a DOT 5.1 fluid in your Guzzi. But I would look deeper than just the rating in big letters. The DOT rating for brake fluid is a minimum rating, "you must be at least this tall to ride this ride" type thing. There could be DOT 4 fluids out there that are every bit as good or better than DOT 5.1 fluids.

In my opinion, cost is the only reason not to use DOT 5.1 fluids.

http://www.stoptech.com/technical-support/technical-white-papers/brake-fluid

 

But I do agree that if your rear brake is fading something my be wrong. As mentioned, we do not have that issue. And we are not THAT slow.

[Scud]

I'm not having a problem with the rear brake - but recently saw Rox lose his on a long, steep downhill, and Todd said he loses his all the time.

 

For me this is just a preventative thought, because I ride of a lot long, steep roads. My question is also partly prompted by the fact that I have an unopened bottle of Motul 5.1 and nothing to use it on (it was specified for the KTM 950 that I no longer have).

 

Re Cost:

I recently bought 365ml of Mopar DOT 4 at Pep Boys for about $9.00 (but same size other brands of fluid can be had for maybe $6.00)

500ml bottle of Motul DOT 5.1 is on Amazon for $11.00-$12.00

Cost/ml doesn't appear to be a significant factor.

[Kiwi_Roy]

Interesting topic.

As to brake fade, I haven't experienced it on my VII but with the tiny calliper I can see it happening for sure. I used to get it all the time in the Ford F150 driving over the mountains in PNG.

 

 

Sent from my shoe phone!

[GuzziMoto]

The size of the caliper has little to do with fade. That is more a matter of disc size, pad used, and fluid used. Condition and function of the brakes can also play a major role in fade. Brakes that drag can easily overheat and that can cause fade. But the caliper is not a heat sink in a disk brake system. How big the caliper is does not really affect how much heat the system can handle.

 

The cost to go from DOT 4 to DOT 5.1 can be a matter of a few dollars. It depends on what fluid you choose. All brake fluids are not created equal. They may all meet the same basic DOT standards, but that is where the similarities end. A high quality DOT 5.1 fluid can be pretty expensive, with some fluids going for $20 a liter or more. And I see people who are determined to save a dollar or two on their oil filter, so it would not surprise me to see people who don't want to spend more on brake fluid than they need to.

But those high end fluids aside, I would have no issue using a DOT 5.1 fluid. I think the point being made was that it should not be required to use DOT 5.1 fluid. If you have an issue with brake fade something is likely wrong. Using DOT 5.1 to cover up an issue with your brakes is a band-aid, and a poor choice at that.

[Scud]

Well stated GuzziMoto. I think I'll go ahead try the 5.1. For me, it's just seems like a good idea to have a little extra safety margin  (higher boil point) on long, mountain descents where the brakes get a serious workout. Those sections probably see similar heats as some race applications.

[Chuck]

Just a heads up, and I'm certainly no expert on brakes..but when I rescued Rosie (Rosso Corsa) the rear rotor was blue, and seriously warped. The rear brake line had gotten out of the clip that holds it, and had been rubbed through, to boot.

Decide it was time for a new rotor, caliper, brake line, and rebuilt the master cyl.

Went for a ride, came back, and the rear rotor was hot. (!) I seldom use it.

Hmmmm.

One trick I use is to hang a 20 lb. weight on the brake pedal overnight, and I hadn't done that yet. That gets all the micro bubbles of air to go into the master. Rode it the next day, and it was fine. Don't ask me why, but I've read of this problem on the spineys, and it cured it for me.

[GuzziMoto]

That is some good advice, Chuck.

I have heard of more than a few people with dragging rear brakes on their V11.

Sometimes it is bleeding, sometimes it is dirt / crud around the slave piston. It could also be from poor adjustment of the linkage not allowing fluid to flow back into the reservoir as it heats up. That could also result from the riders foot applying just enough pressure on the brake lever to move the master past the point where it is open to the reservoir. It could also be from old fluid with water in it.

I typically ride with the balls of my feet up on the pegs, my foot isn't able to apply pressure to the brake lever unless I move my foot to a different position. Then, when I am done I put it back.