DOT Brake Fluids

Some facts about brake fluids.

DOT 5 brake fluid is a silicone-based fluid and is not compatible with DOT 4. It is also less suitable than DOT 4 for ABS systems, because of its higher compressibility.

DOT 5.1 brake fluid is a low-viscosity, high-temperature fluid compatible with DOT 4 and DOT 3. Its low viscosity makes it ideally suited for ABS systems, where it helps the system modulate the braking pressure more easily. Its higher wet and dry boiling points should make it more resistant to brake fade under heavy use. I have had no problems with the braking system since installing this fluid.

DOT 5 and DOT 5.1 are very different standards, confusingly named by the US Department Of Transportation. Of the two, only DOT 5.1 is compatible with DOT 4. Do not use DOT 5 fluid unless you have a good reason and know how to purge the system of all DOT 4 fluid before hand (this usually requires complete dismantling of the braking system and cleaning of the individual components with a suitable solvent). Mixing DOT 5 fluid with any traces of DOT 4 will form precipitants that will clog the braking system.

Harley sells DOT 5 Brake Fluid with the following disclaimer: "Fits all models (except '05-later Touring models, '06-later VRSC™, Dyna®, Softail® and '07 Sportster® models)."

Harley sells DOT 4 with this disclaimer: "Fits '05-later Touring and '06-later VRSC™, Dyna® and Softail® and '07 Sportster® models."

Here's an interesting read on the fluids.

D.O.T. 5 Brake Fluid

DOT 4, like DOT 3 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based). Fluids such as DOT 4 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance by drastically reducing its boiling point. In a passenger car this is not much of an issue, but can be of serious concerns in racecars or motorcycles.
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DanX · Apr 19th, 2009, 05:20 PM

This helps explain why I have strange objects in my reservoir. I paid the local HD dealer to change my tires and brake pads last year. I didn’t like the job they did since my bike was a caddy when I dropped it off and gradually became a very rough ride. I recently changed my tires and replaced my rear wheel with a new stock laced one and during the process found I need to flush my brake system. I would guess that the service tech used the incorrect fluid when he did my brakes. I wouldn’t have guessed that Harley used anything other than DOT 5.

One more reason not to let them service my HOG anymore.

glider · Apr 19th, 2009, 06:45 PM

Just because someone works at a harley dealer doesn't mean they know what they are doing.

That system has to be disassembled and cleaned out thoroughly.

Jubal · Jun 25th, 2009, 03:20 PM

I will be changing the master cylinder and switch housing on my 2006 fat boy, the master cylinder reads very clearly to use DOT 5 brake fluid but the HD dealer says I could use DOT 4. If this was the case why does HD put on the cover to use DOT 5 ?. I think it would be correct to use what the manufacturer says and not the dealer.

glider · Jun 26th, 2009, 07:25 AM

Quote:

Originally Posted by Jubal

I will be changing the master cylinder and switch housing on my 2006 fat boy, the master cylinder reads very clearly to use DOT 5 brake fluid but the HD dealer says I could use DOT 4. If this was the case why does HD put on the cover to use DOT 5 ?. I think it would be correct to use what the manufacturer says and not the dealer.

It's your bike but the dealer doesn't know what they are talking about.

NEVER mix fluids in a system and if you did replace the entire system including lines and calipers, the next person would probably mess it up then not knowing what was in there.
Stay with what should be in there as it came from the factory and find a dealer that knows what they are talking about.

4 and 5 DO NOT MIX!

NEWHD74FAN · Sep 24th, 2009, 12:56 AM

As Glider said it DOT 5 is silicon based and is not compatible with DOT 3,4 or 5.1 PERIOD...the Silicone fluid "settles out and goes to the bottom in globs" which means it lies in the lowest parts of the braking system when mixed and will not just "drain out with simple bleeding" AND it sticks to everything...like vinegar and oil. Only thing to do if it contaminates your system is power flush or to be sure, replace all rubber items it came in contact with...seals, o-rings and hoses...in the caliper(s) and master cylinder.

IF you DO NOT REPLACE those contaminated part, you will be risking a latent brake failure or intermittant braking which is DANGEROUS!

joeysamm · Sep 24th, 2009, 04:49 PM

Hi everyone

I have had issues with my rear brakes for some time

I'll bleed them and the pedal is firm for about a couple of weeks or around 150-200miles. then gradually the pedal starts to get soft

I have replaced the master and flushed the fluid completely...but still the same problem any Ideal what's going on??

glider · Sep 24th, 2009, 05:03 PM

Could be the caliper needs to be cleaned so the piston works free.

Brake Caliper Maintenance - Harley Davidson Community

joeysamm · Sep 24th, 2009, 11:19 PM

I forgot to mention I replaced the caliper as well.

Apr 19th, 2009, 05:20 PM	#1
DanX New Family Ride: 2004 FLSTC Join Date: Apr 19th, 2009 Posts: 2	Re: DOT Brake Fluids This helps explain why I have strange objects in my reservoir. I paid the local HD dealer to change my tires and brake pads last year. I didn’t like the job they did since my bike was a caddy when I dropped it off and gradually became a very rough ride. I recently changed my tires and replaced my rear wheel with a new stock laced one and during the process found I need to flush my brake system. I would guess that the service tech used the incorrect fluid when he did my brakes. I wouldn’t have guessed that Harley used anything other than DOT 5. One more reason not to let them service my HOG anymore.

Apr 19th, 2009, 06:45 PM	#2
glider Community Liaison Ride: 07 Road King Classic Join Date: Jun 21st, 2007 Posts: 21,063 My Mood:	Re: DOT Brake Fluids Just because someone works at a harley dealer doesn't mean they know what they are doing. That system has to be disassembled and cleaned out thoroughly.

Jun 25th, 2009, 03:20 PM	#3
Jubal Start The Engine Ride: 2006 Fat Boy Join Date: Jun 25th, 2009 Posts: 20 My Mood:	Re: DOT Brake Fluids I will be changing the master cylinder and switch housing on my 2006 fat boy, the master cylinder reads very clearly to use DOT 5 brake fluid but the HD dealer says I could use DOT 4. If this was the case why does HD put on the cover to use DOT 5 ?. I think it would be correct to use what the manufacturer says and not the dealer.

Sep 24th, 2009, 12:56 AM	#5
NEWHD74FAN Moderators Ride: 2004 Sportster XL1200RS Join Date: Aug 24th, 2008 Location: Camarillo, CA Posts: 4,962 My Mood:	Re: DOT Brake Fluids As Glider said it DOT 5 is silicon based and is not compatible with DOT 3,4 or 5.1 PERIOD...the Silicone fluid "settles out and goes to the bottom in globs" which means it lies in the lowest parts of the braking system when mixed and will not just "drain out with simple bleeding" AND it sticks to everything...like vinegar and oil. Only thing to do if it contaminates your system is power flush or to be sure, replace all rubber items it came in contact with...seals, o-rings and hoses...in the caliper(s) and master cylinder. IF you DO NOT REPLACE those contaminated part, you will be risking a latent brake failure or intermittant braking which is DANGEROUS! Last edited by NEWHD74FAN; Sep 24th, 2009 at 01:00 AM.

Sep 24th, 2009, 04:49 PM	#6
joeysamm Start The Engine Ride: 2000 FXST Join Date: Sep 12th, 2009 Posts: 23 My Mood:	Re: DOT Brake Fluids Hi everyone I have had issues with my rear brakes for some time I'll bleed them and the pedal is firm for about a couple of weeks or around 150-200miles. then gradually the pedal starts to get soft I have replaced the master and flushed the fluid completely...but still the same problem any Ideal what's going on??

Sep 24th, 2009, 05:03 PM	#7
glider Community Liaison Ride: 07 Road King Classic Join Date: Jun 21st, 2007 Posts: 21,063 My Mood:	Re: DOT Brake Fluids Could be the caliper needs to be cleaned so the piston works free. Brake Caliper Maintenance - Harley Davidson Community

Sep 24th, 2009, 11:19 PM	#8
joeysamm Start The Engine Ride: 2000 FXST Join Date: Sep 12th, 2009 Posts: 23 My Mood:	Re: DOT Brake Fluids I forgot to mention I replaced the caliper as well.

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DOT Brake Fluids

Brakes, Tires and Wheels