I need to get some brake fluid for my split, and I see there are various dot ratings? What should I be looking for, or any recommendations? I'm currently running stock drums front and back...

dot 4

I use silicone brake fluid , doesn't burn the paint off if it drops on body work ,doesn't absorb water , "AUTOMEC "sell it about £20 quid a ltr enough to do splitty , equal to dot5.

Thanks

dougall up wrote:I use silicone brake fluid , doesn't burn the paint off if it drops on body work ,doesn't absorb water , "AUTOMEC "sell it about £20 quid a ltr enough to do splitty , equal to dot5.

I hope you either started with all new components or flushed the brake system completely before using DOT5 fluid.

"Using DOT 5 in a DOT 3 or DOT 4 system without proper flushing will cause damage to the seals and cause brake failure. DOT 5 brake fluid absorbs a small amount of air requiring care when bleeding the system of air."

passion4 wrote:I need to get some brake fluid for my split, and I see there are various dot ratings? What should I be looking for, or any recommendations? I'm currently running stock drums front and back...

Brake fluid... Bit of a mystery topic

There is a degree of confusion regarding the specification of brake fluid and this article sets
out to clarify the situation.
The Department of Transportation (DOT) classifies brake fluids to defined specifications.
These specifications relate to their boiling points and chemical composition, both of which
are important. All currently available brake fluids are covered by one of the following
specifications; DOT3, DOT4, DOT5 and DOT5.1.
The laws of thermo-dynamics dictate that the energy from motion is turned into heat through
friction. A braking system only works efficiently if the fluid remains incompressible. If the
brake fluid boils, it turns to gas, which is compressible and the braking system becomes
“spongy” or in extreme cases fails completely.
A brake system is not perfectly sealed and moisture can get into the system and be
absorbed by the fluid. The effect is to reduce the boiling point of the fluid, which reduces the
efficiency of the braking system, as described above.
The DOT specifies two reference tests for brake fluids.
* Dry boiling point - the boiling point of fresh fluid
* Wet boiling point –the boiling point once the fluid has absorbed moisture
(representing brake fluid after time spent in a real situation).
There are two main types of brake fluids:
* DOT 3, DOT 4, Super DOT4* and DOT 5.1 which are based on poly glycol
compounds.
* DOT 5, which are based on Silicone.

Note the two types of fluid are not compatible and must not be mixed in
a braking system.

SILICONE BRAKE FLUID (DOT 5)
Silicone based DOT 5 was originally introduced to give higher temperature performance over
glycol DOT 4. Silicone fluid also has other advantages, it does not damage paintwork and it
does not absorb water.
However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG
fluids. It is also more compressible than PAG fluids, which can result in a sluggish or spongy
pedal. It therefore requires special design considerations in braking systems. Further,
because it does not absorb water, any water remains as globules, which can pool in low
spots in the system and cause corrosion. This water can vaporise when heated under heavy
braking giving a disastrous effect on braking efficiency.
DOT5 fluids are not recommended for motor sport applications.

POLY GLYCOL BRAKE FLUIDS (DOT 3, 4 AND 5.1)
Glycol based DOT 4 fluid is the current mainstream brake fluid, and you will see that the
specification is considerably better than DOT 3 which it replaces.
DOT 5.1 has higher specification still and is for fast road and occasional track day use. It has
a similar spec to DOT4 for the boiling point (>260) but is a lot lower viscosity @-40C typically
900 centistokes (compared to 1500 - 1800 centistokes for DOT 4 and super DOT 4).
Listed in the table below, are the minimum dry/wet boiling point specifications for each DOT
level.
BOILING POINT:
DOT 3 - 205°C (dry) / 140°C (wet)
DOT 4 - 230°C (dry) / 155°C (wet)
DOT 5 (silicone) - 260°C (dry) / 185°C (wet)
DOT 5.1 (PAG) - 260°C (dry) / 185°C (wet)
Super Dot4 * - 300°C (dry) / 195°C (wet)
(racing brake fluid)
* Super DOT4: The main difference between DOT 4 and Super DOT 4 is the dry boiling
point. Normal Dot4 is >260C whilst Super DOT 4 is more like >310C
With thanks to Martyn Mann - Technical Director Millers Oils.

Excellent post - thanks Jim

Thanks Jim for taking time out to pull that all together, much appreciated! Everyday is a school day!

So Dot 5 cannot be mixed with Dot 3 or Dot 4, but can Dot 3 or Dot 4 be mixed? I only ask as how do you tell what you currently have in a bus if you have not installed a new system from scratch?

DOT 3, 4 & 5.1 are all the same generic solution so are compatible. Naturally if you have 50% DOT 3 and the rest DOT 5.1 you will get a performance level somewhere lower than DOT 5.1

DOT 5 is completely different, hence needs to be kept on its own.

All you can do is ask the previous owner. But virtually everyone will be using DOT 4