I bit the bullet and took apart the Fox hydraulics for a complete rebuild (it's a covid lockdown thing I suppose).
I was running Dot 4, re-bled about every 4 years. I see some fluid discolouration and feel a bit of rust 'grit' in the clutch master, oddly not coming down from the reservoir as I first thought. The inside of that is perfectly clean.
I am rebuilding the cylinders with all new rubber so now is the time to carry on with Dot 4 or go to Dot 5.
The quote below is from the web...I'd like to use my same lines, but what do I flush them with? Is Dot 5 the end of my worries that will outlast the vehicle? Dot 5.1?
Quote:
DOT 4 was used by many British or European cars and is also a poly-glycol base. DOT 4 absorbs less moisture than the DOT 3 fluid, and many enthusiasts have converted their hydraulic brake systems from DOT 3 or DOT 4 for this reason. Changing the fluid is not all that is involved in this conversion, though. To prevent cross contamination you should also change all the rubber components in the brake system. All steel lines and brake hoses need to be flushed, too. DOT 4 has a boiling point of 311 degrees F., and it is rated at a viscosity of 1800 at -40 F.
DOT 5 is silicone-based brake fluid and is used in most new cars today. DOT 5 is expensive, but it has a dry boiling point of 356 degrees. Newer brake rotors tend to be smaller and thinner, which means they disperse heat a lot less efficiently. Also, DOT 5 does not absorb any moisture. DOT 5 will not harm painted surfaces and acts as a weather barrier for your brake system, preventing rust. DOT 5 can be used as an upgrade or replacement for both DOT 3 and 4, but should not be mixed with any of the other fluids. The procedures to convert your DOT 3 or 4 systems to the silicone-based DOT 5 are similar to the Dot 4 conversion procedure mentioned above. Silicone-based brake systems tend to be more difficult to bleed, but once this is accomplished, a DOT 5-filled system will resist rust better and last longer than the other formulations. Silicone brake fluid is also much lighter in cold temperatures, only about 900 weight at -40 F.
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