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Originally Posted by Camfab
It’s not about air being dissolved into the fluid, though at a high enough pressure it would.
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I understand that, but that's what I though meant when you wrote:
"The tanks like the Motiv set up are actually introducing air into your precious fluid." My reply was that they aren't introducing any more air than is already being introduced during the bleed/flush process anyway.
Quote:
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It’s about relative humidity and the hygroscopic nature of brake fluid. Think of it this way, when you use your compressor you are just compressing outside air. What happens, you end up with a ton of water. Now that’s an exaggerated example because you’re moving a large volume of air. At 75% humidity at Atmospheric pressure the air above the brake fluid contains 75% of the water that a given volume of air can hold before it precipitates into water drops. Now taking that same air and pressurizing it increase the likelihood that the moisture will go from a gas to a liquid. Knowing that brake fluid loves water you can guess what happens next. It’s something to consider when you are looking for the absolute best performance from your brakes. This is why the best and admittedly biggest hassle pressure bleeders isolate the fluid from air with a diaphragm.
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The humidity of air is about how much water the
air can hold. It doesn't mean that water is going to somehow transfer into your brake fluid: it's already absorbed into the air! The only way this is going to hurt anything is if the water condenses and falls out of the air and into the brake fluid (consdensation). This is determined by the relative volume of the air at ambient vs a compressed state. But a Motiv bleeder doesn't operate at high enough pressures (15psi) to cause condensation. If it did, you'd literally see condensation droplets all over the inside walls of the jug, and that doesn't happen. OTOH, an air compressor that usually operates at over 100psi
can cause condensation, and one only has to drain one's compressor tank to see that. That's why those professional systems need a bladder.