Brake fluid question

cwtoyota

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cwtoyota 5/1/2019 11:28am

5/1/2019 11:28am Edited Date/Time 5/1/2019 11:28am

VetMX.com wrote:

My RM250 has a mushy back brake so I was going to bleed it and add some new fluid. What type of fluid does it take...

My RM250 has a mushy back brake so I was going to bleed it and add some new fluid. What type of fluid does it take stock? Type 4?

Jap bikes are all DOT 4 stock. You can mix 3 and 4, but you can't mix those with DOT 5.
All the OEM fill-caps have a label on them saying which fluid they suggest.

If you want to switch to DOT 5, make sure you completely disassemble and clean the master and the caliper.

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Falcon

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Falcon 5/1/2019 11:40am

5/1/2019 11:40am

To add to cwtoyota's post, I would never switch to DOT5 on a motocross motorcycle. DOT5 is great for cruisers because it won't etch the chrome if it was ever spilled. It is also very hydrophobic, meaning it won't absorb water like DOT 3 or 4 will. Finally, it has a higher boiling point than DOT 3 and 4. However, it will not mix with DOT 3/4, so you must COMPLETELY clean out every last drop of that old fluid to avoid losing your brakes altogether.

If you want the benefits of a higher boiling point, go with a DOT 5.1, which will mix with 3 and 4. IMO, this is unnecessary in motocross applications. Save some money and get a good DOT 4.

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77Moto

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77Moto 5/1/2019 11:56am

5/1/2019 11:56am

Falcon wrote:

To add to cwtoyota's post, I would never switch to DOT5 on a motocross motorcycle. DOT5 is great for cruisers because it won't etch the chrome...

To add to cwtoyota's post, I would never switch to DOT5 on a motocross motorcycle. DOT5 is great for cruisers because it won't etch the chrome if it was ever spilled. It is also very hydrophobic, meaning it won't absorb water like DOT 3 or 4 will. Finally, it has a higher boiling point than DOT 3 and 4. However, it will not mix with DOT 3/4, so you must COMPLETELY clean out every last drop of that old fluid to avoid losing your brakes altogether.

If you want the benefits of a higher boiling point, go with a DOT 5.1, which will mix with 3 and 4. IMO, this is unnecessary in motocross applications. Save some money and get a good DOT 4.

This is true EXCEPT Dot 4 RBF fluids have a much higher boiling point than 5.1.

This info is readily available anywhere they sell RBF.

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WoodsRacer

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WoodsRacer 5/1/2019 4:30pm

5/1/2019 4:30pm

VetMX.com wrote:

My RM250 has a mushy back brake so I was going to bleed it and add some new fluid. What type of fluid does it take...

My RM250 has a mushy back brake so I was going to bleed it and add some new fluid. What type of fluid does it take stock? Type 4?

cwtoyota wrote:

Jap bikes are all DOT 4 stock. You can mix 3 and 4, but you can't mix those with DOT 5. All the OEM fill-caps have...

Jap bikes are all DOT 4 stock. You can mix 3 and 4, but you can't mix those with DOT 5.
All the OEM fill-caps have a label on them saying which fluid they suggest.

If you want to switch to DOT 5, make sure you completely disassemble and clean the master and the caliper.

DOT 3, 4, and 5.1 can all be mixed since they are all glycol based. DOT 5 is silicone based and should never be mixed with any other brake fluid or any other brake fluid system. Glycol vs silicone based systems will use different materials and seals that will not live with the other type of fluid.

As it comes to temperatures not all brake fluids of the same DOT rating are equal. DOT just sets min requirements on boiling, as such some fluids meet the min spec and some strive to be market leaders. Here is a great link comparing fluid types. Brake Fluid Comparison Table

I run Motul RBF600, price is reasonable, wet boiling point is plenty high, and most motorsports places stock it. Also just as a note, I typically always compare brake fluid wet boiling points. Unless you change the stuff regularly all glycol based fluid will pull in moisture, so the dry boiling points are just fancy numbers, the true system degrade at the wet boiling points.