Brake Fluid?

[rgoot01]

I have a question that hopefully can be answered here regarding types of brake fluid. My buddy is building a CB750 and he has an uncle who will give him some brake parts of a H-D. My concern is that the H-D parts are designed for DOT 5 fluid yet the CB system used DOT 3. I have read the thread regarding brake fluid in the maintenance section, however can't get a definitive answer.

My question is, if one were to completely rebuild the brake system; calipers,masters,and new hoses; would my friend be able to run either of the three DOT 3,DOT 4,or DOT 5 as long as he stayed with that fluid?

I know that some natural rubbers will deteriorate if they come in contact with silicon and petroleum products, but will DOT 3 or 4 deteriorate rubbers designed for silicon fluids?

[natemoen]

It could, but the seal and rings may be designed to withstand both. You would need to find out what its meant to withstand.

[James England]

I had a similar problem establishing what fluid to use. I use dot 5.1 on my rebuilt brakes. The way I see it... the hoses are new, the caliper seals are new, the master cylinder seals are new..... and they are most probably made of an entirely different substance from that used in the 1970's and 80's. I doubt they have any rubber in at all these days. Same goes for O rings and things.

I've had Dot 5.1 in mine for a year and I recently drained the fluid and checked everything. There is no degradation of any parts that I can detect. No leaks, no soft seals. That's after a year. So, I intend to continue using it.....

[tinman905]

You can run Dot 5 if you have a complete new system but it will gel up if there was Dot 3 or Dot 4 in there first.
You would never want to use a Dot 3 or Dot 4 in a system that was intended for Dot 5 ,then you would find the 3 and 4 eating the seals.

Unless DOT 5 is recommended by the brake manufacturer and comes with it new, just run the highest boiling point glycol-based fluid you can find. It's cheap and it'll perform better and all you have to do is change it once or twice every couple years or so.

There is a Dot 5.1 that can be used with Dot 3 or Dot 4 ( all three glycol-based) but never use it with Dot 5 which is silicone based.

You might find Dot 5 a little spongy compared to the Dot 3 or 4. There are a few guys here using Dot 5 and like it , but it's like comparing tires ,oil and air pressure everyone will have their own opinion.
Mine is go with what the system was designed to use.

[crazy steve]

Harley has gone back-and-forth between DOT 3, 4, and 5 fluid in the last 15 years or so, but the rebuild kits for their brake components haven't changed so I'd say you won't have any worries with the seals. Just don't mix DOT 5 with anything else...