Advice on Swap to Dot 5

Advice on Swap to Dot 5 - New MC

jos51700 said
I think we're agreeing and not realizing it.

What happens when you add DOT 5 to a DOT 3 system? Nothing abnormal! Don't believe me? Ask Harley Davidson! They switched from DOT 5 to DOT 3 in the late 2000's. The only part numbers in their system that changed were the master cylinder lids, because they indicated the change in spec fluid. All the seals and other parts were unchanged.
Plenty of mouthbreathing HD guys dumped in DOT5 without realizing that there had been a change. No problems! (Note, these are the same guys that think the fluid is being used, like engine oil, so they top it off, not realizing why they can't get the pistons to retract when it's time for new pads)

Rick E-berg has always advised that it's ok to mix DOT 3 and DOT 5. The characteristics that are different have no effect on the functionality of the fluid.

Ask Penrite why they're wrong!

It wasn't from mixing DOT 3/4 and 5. In 2004 I mixed all three in a baby food jar, and even punched a hole in the lid so it could absorb some moisture. I finally threw it away a couple years go. No gunk.

This person gets it.

My personal opinion about why they say not to mix is twofold.
#1, it's difficult to define what you can put in the system, and saying what can mix makes that more unclear. Mineral oil brake fluid, which is a thing, should never be run in a 3/4/5/5.1 system. The materials in the cups and seals is totally different.

#2, Manufacturers have to dodge any liability arising from unclear instructions.

Actually we disagree on the logic and therefore the conclusions.
1. Key difference is that I conclude that it is inadvisable to mix SBBF DOT5 into a system with DOT3, 4, or 5.1 type fluids or vics versa.
2. Our logic is different because our understanding of the performance requirements of FMVSS is different. These are the minimum requirements for each of the fluids - fluids defined by industry standards (SAE) - and not design guidelines or advice for use in a system.
3. Additionally the weight we each place on manufacturer recomendations differ. I do not dismiss them as simply legal stratagem. For a change to be acceptible to an automobile manufacturer the component would have to pass both design and production validation testing. Those tests usual involve both component and full system testing, in typical and extreme environments, and in sufficient quantity to pass those tests 100% of the time statistically at 6 sigma (or more!). (sigma = standard deviation)

If Harley davidson has a tech bulletin stating the SBBF may be added to the systems already containing non-SBBF without a complete fluid change over, then its probably fine to do on those systems.