Quote:
Originally Posted by Msquared
There is literally air above your brake fluid in the reservoir at all times, as there is when you open the bottle and pour the new fluid in to whatever you pour it into. Therefore, there's no "introducing air" into the fluid with a pressure bleeder - they've known each other since the fluid was manufactured! Brake fluid doesn't absorb air anyway. That's just not a thing.
Finally, I don't think these reservoirs are as tightly sealed as you believe they are. If they were, then the fluid level inside them could never drop because no air could get in to replace the lost fluid; which would obviate the entire purpose of the reservoir. They are sealed enough to keep large amounts of moisture out, but that's about it. But no matter what method you use to bleed your brakes, the fluid is going to be exposed to air with moisture in it for most of the duration of the process. There's probably a good argument that the pressure bleeder is actually keeping more moisture away from the fluid during a bleed/flush process, since it forms a semi-sealed system with the fluid instead of just leaving the reservoir open.
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Matt every vented cap I’ve ever encountered has had a rubber bladder below it. The vent allows the bladder to move, allowing the movement of fluid. It’s not about air being dissolved into the fluid, though at a high enough pressure it would. 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.