Brake Fluid Exchange

My question regards to preventative maintenance of the fluid portion of the braking component of a car with ABS. The first part of my question is does the brake fluid in the master cylinder reservoir over time inter mingle with the rest of the fluid in the system? If the answer is yes, does it make sense to remove the fluid in the reservoir and replace it with fresh fluid at every oil change? As I do all my own preventative maintenance the expense would be minimal. I’m old school and follow the doctrine of do no harm and figure why risk the chance of opening the system to do a conventional bleed, not to mention the mess.

I cannot imagine that the fluid in the master would circulate to the calipers, but your interest in doing maintenance suggests that you would probably go the extra mile here, so why not change the fluid in the master and then bleed all four wheels, pumping the old fluid out and replacing it with fresh? Once a year would be plenty, that’s what I do and it works fine. Good luck! Rocketman

Fluid should be flushed every three to five years. Some like to do it with every brake job. Unless you have some sort of malfunction, don’t mess with the master cylinder. Flushing is best done by pushing fresh fluid in at the master cylinder and drawing the old fluid out at the calipers.

To answer your first question, the fluid in the MC flows down through the valves in the ABS modulator and into the calipers. The fluid does not intermix throughout the system in any significant way. The fluid in the calipers will not find its way back to the master cylinder. The most contamination in the system will be at the calipers, but it will cannot move back up into the system in any significant way unless when changing pads someone pushes the pistons back without opening the bleeders. Even then, problems are rare, but every procedure you’ll find for ABS equipped systems will recommend against it to prevent contamination from getting pushed back into the ABS modulator valves, small solenoid-operated needle valves that effect the ABS action.

Circulate, no, diffuse, yes. The crud in the calipers pretty much stays there. Any moisture in the brake fluid will diffuse throughout the system. Water mixes with brake fluid and it can work its way in right in the strangest of ways. Through the rubber brake lines and in the master cylinder vents.

That’s why @rocketman suggests bleeding the system regularly. Get the master cylinder fluid to push the crud out (not back up into the ABS valves) and clean, dry fluid to take its place. If you are rotating the tires, bleed the system then. If you are changing brake pads, bleed the system then. Do it periodically as you work on other things and you’ll prevent a lot of internal corrosion that occurs if the fluid gets old and wet.

The inter mingling concept was a result of the observed discoloration that appears in the reservoir which would imply that the crud is working it’s way back to the resevoir. I realize and agree that on a badly contaminated system the only solution is as has been suggested by the replies. I was thinking and probably should have said I was thinking of doing this on a brand new vehicle and at 5K intervals to stay ahead of the contamination. I don’t know for sure but based an past flushes I’ve done at the wheel cylinders, it appears approximately one third of the fluid in the system resides in the reservoir.

Exactly. Some folks think that the water collects in the calipers because the fluid looks dirty when bled. That’s not water, that’s dirt and corrosion. The water is dissolved and distributed throughout the system.

I would think the fluid would thermosyphon to some extent. The MC is higher than the brakes; the hotter fluid near the calipers would migrate, at least a little bit, to the MC.

That said, I’ve noticed truly gunky fluid coming out of never-opened bleeders. Every oil change is excessive, IMO…but every brake job: you’re supposed to bleed the brakes anyways, why not bleed 'em until you’ve flushed the lines?

For the most part I agree with all the above. Brake fluid is hydroscopic which means that moisture will be absorbed by the brake fluid and moisture will migrate from wet to dry, that is it will distribute itself throughout the system by diffusion the same way that if you take a dry sponge and wet one end, before long the water at the one end will have evenly distributed itself throughout the sponge.

As the water reaches other parts of the system, it will begin to corrode the metal.

As the system is pretty well sealed except for a small vent in the MC cap, it takes a long time for a significant amount of moisture to accumulate, so changing the fluid in MC at every oil change is a bit overboard. I usually flush out my brake system at every pad change unless the owners manual prescribes a different schedule.

When I do change it, I suck out as much of the brake fluid from the MC as I can get and then refill it. Then I attach some clear vinyl 1/8" fist tank tubing to each bleeder and loop it up over something that is at least 2" above the bleeder and down to a bucket or jar. I sometimes do two or even all four bleeders at the same time to speed things up. If I do two at a time, I do the back brakes first. I just open the bleeders and let gravity do its job. I keep the MC reservoir full.

Once all calipers and wheel cylinders are flowing clear, I close the bleed screws, do a final top up and remove the tubing and containers. If the MC never goes dry, then no air gets into the system and pressure bleeding is not needed. Since no one is stepping on the brake pedal, the ABS is not affected, but it also does not get flushed either. I don’t think the small amount in the ABS will do any harm, it will cycle back to the MC eventually. Just to be sure, unplug the ABS connector while flushing.

edit: One last comment, the level of brake fluid in the MC reservoir is an indicator of the amount of life left in your brake pads. As the pads wear down, the caliper piston moves out to take up the space and keep the brakes adjusted. This draws fluid from the reservoir. As the pads reach their minimum, the brake fluid level also drops to the min mark. If you are always topping off the brake fluid, or replacing the fluid in the MC and keeping it full, then you cannot monitor the wear of the pads.

If you still want to periodically change the brake fluid in the MC, it wont hurt. The fresh brake fluid will draw some of the moisture from the rest of the cylinder under the same principles that got it there in the first place. But mark the fluid level in the MC before you draw out the old brake fluid and refill only to this mark. Use an erasable marker for this.

I do it once a year, nice weather, when I’m doing an oil change or tire rotation. Of course with a brake job or something else, that would be part of the job. That’s why I said I think the OP is probably going to go the extra mile, sounds like a heads up guy to me. I consider it part of maintenance and feel a lot better after doing it, so it’s good for my car AND good for my head. Rocketman

“…the observed discoloration that appears in the reservoir…”

This is caused by the slow degradation of the black rubber seal cups in the master cylinder. When mine gets a little dark, I suck it out with a turkey baster and replace.

Yes, the fluid is hygroscopic.