Dot 3 to Dot 5 Brake fluid
Seasoned HDF Member
Joined: May 2012
Posts: 32,934
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From: Carlsbad, CA
Id make sure all the components are compatible with DOT 5.
Interesting from Wikipedia:
DOT 3, like DOT 4 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5, which is silicone-based). Fluids such as DOT 3 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance, and if allowed to accumulate over a period of time, can drastically reduce its boiling point. In a passenger car this is not much of an issue[citation needed], but can be of serious concern in police vehicles, racecars or motorcycles[citation needed].
DOT3 has been all but replaced with the superior DOT4 as there is little cost difference between the two.
Boiling points
Minimal boiling points for these specifications are as follows (wet boiling point defined as 3.7% water by volume):
Boiling point ranges [2]
Dry boiling point Wet boiling point
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 260 °C (500 °F) 180 °C (356 °F)
DOT 2 Brake fluids DOT 4
Interesting from Wikipedia:
DOT 3, like DOT 4 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5, which is silicone-based). Fluids such as DOT 3 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance, and if allowed to accumulate over a period of time, can drastically reduce its boiling point. In a passenger car this is not much of an issue[citation needed], but can be of serious concern in police vehicles, racecars or motorcycles[citation needed].
DOT3 has been all but replaced with the superior DOT4 as there is little cost difference between the two.
Boiling points
Minimal boiling points for these specifications are as follows (wet boiling point defined as 3.7% water by volume):
Boiling point ranges [2]
Dry boiling point Wet boiling point
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 260 °C (500 °F) 180 °C (356 °F)
DOT 2 Brake fluids DOT 4
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Joined: Mar 2010
Posts: 38,586
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From: Honah Lee
The reason they went to 3 or 4 was ABS system where not designed for 5. So Harley did it across all models. On a motorcycle, nothing beats DOT 5 . Does not absorb moisture or hurt paint. Still need to change it ever few years by vacuum pump.. Never pump master.
Because DOT 5 cant be used with ABS and manufacturers buy in bulk.
Last edited by Ed Ramberger; Feb 3, 2018 at 01:07 PM.
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3rd Gear
Joined: Feb 2018
Posts: 7
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From: Texas
Dot 5 because it's silicone base, it doesn't absorb water and is not harmful to paint or anything that I know of. My 89 and 2001 use Dot 5. I know it can't be used with ABS, I think it might have something to do with the rubber components (not sure).
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3rd Gear
Joined: Feb 2018
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From: Texas
Why can't you pump the master cylinder to bleed the brakes? I have a spongy front brake on my 2001 RK and Have bleed it both ways, (hand vacuum pump and pumping the master cylinder), with no improvement. Have to pump it twice to have a good front brake.
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Joined: Jun 2007
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From: Bedford UK
BT, I would only swap to DOT5 if you have stripped and cleaned your entire system, preferably using new rubber components throughout. I've done that with DOT4 calipers. An alternative is to use DOT4, which my other two bikes use. Despite the comments made about DOT5, the recommendation is the same as for DOT4 vehicles, to change every 2-3 years.
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Joined: Mar 2010
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From: Honah Lee
I
You can pump it. Obviously, you have air in the system or the seals are worn out and it has already gone past normal movement.
Why I said do not pump the master was the normal refill with new fluid and to get moisture out. This is not so mandatory on DOT5 but the new ABS system MUST BE CHANGED OUT EVER TWO YEARS..at least Harley's system.
When all is OK when braking, the master only moves so far and leaves a wear and crud build up at the end of the normal movement. If you break the bleed on the caliper and push the master thru that wear and crud edge, there is a good chance, it will screw up the lip seal of the master cylinder.
Also to add further confusion now, keep in mind the caliper seal is a square or sometimes quad seal. It will withstand pressure either way. The master is a lip seal. Pressure one way towards the caliper. The other way is very easy to leak by. So when you pull a vacuum on the caliper to pull fluid out, go slow and only about 5 lbs of vacuum. Pulling a whole lot will pull air laden with moisture into master past that seal.
Why I said do not pump the master was the normal refill with new fluid and to get moisture out. This is not so mandatory on DOT5 but the new ABS system MUST BE CHANGED OUT EVER TWO YEARS..at least Harley's system.
When all is OK when braking, the master only moves so far and leaves a wear and crud build up at the end of the normal movement. If you break the bleed on the caliper and push the master thru that wear and crud edge, there is a good chance, it will screw up the lip seal of the master cylinder.
Also to add further confusion now, keep in mind the caliper seal is a square or sometimes quad seal. It will withstand pressure either way. The master is a lip seal. Pressure one way towards the caliper. The other way is very easy to leak by. So when you pull a vacuum on the caliper to pull fluid out, go slow and only about 5 lbs of vacuum. Pulling a whole lot will pull air laden with moisture into master past that seal.
Last edited by Jackie Paper; Feb 3, 2018 at 02:36 PM.