Hydraulic reservoir

[Lynn Eades]

What about the use of a silicone fluid? Would this help you bypass the obstacle?

[Bruce Parker (RIP)]

Originally Posted by Lynn Eades

What about the use of a silicone fluid? Would this help you bypass the obstacle?

I can't recall my original decision to use non-silicone fluid (something about a promise to re-bleed annually to avoid moisture buildup I expect) and to be fair the DOT3 and DOT4 fluid has done me well for at least 10 years now. But for the current maintenance it's the rust in the reservoir that's filtering down to the clutch and accelerator master cylinders that's giving me pause.

[Lynn Eades]

Ok then. Somehow you need to stop the moisture getting through the cap vent. Can you modify it into a closed system? Some vehicles have a master cylinder with a larger lid that has a flexible diaphragm contained within. This moves with the fluid level, but is completely sealed.

[charlie fitton]

What is the volume/shape of the tank?

Keeping it totally filled, and maybe a non vented cap for storage....

[Hanno Spoelstra]

I'd fit a new cap to the master brake cylinder with a fitting and tap into a plastic fluid reservoir fitted over the master brake cylinder. That way you can keep the steel master cylinder full reducing the risk of rusting the inside of the master cylinder as long as you replenish the brake fluid on a regular basis.

Makes sense?

brake fluid reservoir.jpg

[Bruce Parker (RIP)]

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?

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.

[Lynn Eades]

Bruce, as an apprentice we were taught to use methalated spirits to clean brake components. This would be when you have them stripped. You would then lube components with the brake fluid that you intend to use, on assembly. The following, I ripped from the net:

Since DOT 4 fluids were developed, it was recognized that borate ester based fluids offered the potential for boiling points beyond the 446F requirement, thus came the Super DOT 4 fluids - some covered by the DOT 5.1 designation - which exhibit a minimum dry boiling point of 500F (same as silicone, but different chemistry).

You can go ahead and mix or flush (replace) any DOT 3, DOT 4, or DOT 5.1 brake fluids, no problem. So there are no special bleeding procedures and you don't need to run anything in the lines. The only one to watch out for is DOT 5 (silicone brake fluid).

DOT 5 does not damage paint like other brake fluids do, but is not recommended by AP Lockheed for use in their systems due to higher wear than with other type fluids.

Brake system contamination:

The single most common brake system failure caused by a contaminant is swelling of the rubber components (piston seals etc.) due to the introduction of petroleum based products (motor oil, power steering fluid, mineral oil, etc.) A small amount is enough to do major damage. Flushing with mineral spirits is enough to cause a complete system failure in a short time. I suspect this is what has happened when some owners change to DOT 5 (and then assumed that silicone caused the problem). Flushing with alcohol also causes problems. Brake systems should be flushed only with DOT 3 or 4.

If silicone is introduced into an older brake system, the silicone will latch unto the sludge generated by gradual component deterioration and create a gelatin like goop which will attract more crud and eventually plug up metering orifices or cause pistons to stick. If you have already changed to DOT 5, don't compound your initial mistake and change back. Silicone is very tenacious stuff and you will never get it all out of your system. Just change the fluid regularly. For those who race using silicone fluid, I recommend that you crack the bleed screws before each racing session to insure that there is no water in the calipers.

British Cycle Supply Company does not recommend use of DOT 5 silicone brake fluid on British motorcycles equipped with any AP Lockeed disc brake components.

Not exactly what you were asking, but should help.