Change Your Brake Fluid

[93 525 Paul]

When was the last time?
Found a great thread from 2008 but it went off on a tangent

...on brake fluid...it does need to be changed every 2 years. Your braking performance is really impacted by this.
Just try it on your car... You really need to keep flushing until clean fluid comes out. It doesn't take much fluid to do this job and most shops don't even do it thoroughly enough.

Brake fluid is hydrophilic, the moisture absorbed corrodes the braking system and lowers the boiling point of the fluid. The brown color you see in old fluid is rust from this process.
Everything will last longer if the fluid is uncontaminated. The boiling point is seldom a factor for street driven cars, but try and get a race car past tech with dirty brake fluid.

Changed the fluid last night and the old stuff was rootbeer brown (black really) even thought I've changed it many times over the years and the new a light golden color. To wit:


Imagine what it'd be like if it were never changed. I did it the easy way. Jacked up the car, put it on jackstands, removed all four wheels, mityvac'd all the old fluid from the res, filled with fresh Dot 4 and opened the RR bleeder, put a hose on it and let it gravity flow into a bottle. Moved around the car (LR, RF and LF) and with less than a quart got good clean fluid from each corner.

[bad_manners_god]

When was the last time?
Found a great thread from 2008 but it went off on a tangent




Changed the fluid last night and the old stuff was rootbeer brown (black really) even thought I've changed it many times over the years and the new a light golden color. To wit:


Imagine what it'd be like if it were never changed. I did it the easy way. Jacked up the car, put it on jackstands, removed all four wheels, mityvac'd all the old fluid from the res, filled with fresh Dot 4 and opened the RR bleeder, put a hose on it and let it gravity flow into a bottle. Moved around the car (LR, RF and LF) and with less than a quart got good clean fluid from each corner.

You forgot to compress the caliper pistons as thats where the majority of the dirty brake fluid is

[DueyT]

Like BMG notes, lots of fluid in the pucks. I change the fluid when I replace the pads and I open the bleed valve as I retract the piston into the caliper to get as much of the old fluid out of the caliper body.

Wow, I've never seen brake fluid that dark before!!!

Cheers
Duey

[93 525 Paul]

You forgot to compress the caliper pistons as thats where the majority of the dirty brake fluid is

My expectation is that the old fluid is displaced by the new fluid. It doesn't seem that he new fluid would run clear until the caliper fluid is out. I used a clear tube on the bleeders and didn't close the bleeder until it ran clear. The volume of dark fluid is far greater than the volume of the lines, so it would suggest that the caliper fluid was indeed flushed.

[bad_manners_god]

My expectation is that the old fluid is displaced by the new fluid. It doesn't seem that he new fluid would run clear until the caliper fluid is out. I used a clear tube on the bleeders and didn't close the bleeder until it ran clear. The volume of dark fluid is far greater than the volume of the lines, so it would suggest that the caliper fluid was indeed flushed.

Doesn't work like that, the fluid that is inside the calipers is sitting at a lower level than the bleeder, since air is lighter than fluid it will rise to the top and bubble up which is why the bleeder is at the highest point, as soon as you would have compressed the piston, the rest of the black fluid would come out.

[Ross]

A lot of the crappy fluid will remain in the bores of the calipers. If you crack open a bleeder now after it had a chance to mingle with the new you'll see. The new fluid doesn't circulate through the caliper but takes the easiest path to the bleeder.
Retracting the pistons, bleeding, operate the brakes to re-extend the pistons and repeat will get pretty much all of it.
I usually don't go to the trouble unless the fluid was really crappy looking, like yours was.

[xspeedy]

A lot of the crappy fluid will remain in the bores of the calipers. If you crack open a bleeder now after it had a chance to mingle with the new you'll see. The new fluid doesn't circulate through the caliper but takes the easiest path to the bleeder.
Retracting the pistons, bleeding, operate the brakes to re-extend the pistons and repeat will get pretty much all of it.
I usually don't go to the trouble unless the fluid was really crappy looking, like yours was.

Is there an easy way to perform the retraction of the piston short of removing the caliper and pads (as done when swapping pads)? If not, I also heard it isn't a good idea to apply the brakes with the caliper out for fear of ejecting the piston completely.

I guess I'm asking for the best practice for this job. I flushed my brakes but never cycled the pistons. I'd like to go back and do this.

[Ross]

On the fronts which are floating simply prying the rotor against the caliper. Insert a large screwdriver into the vent holes of the rotor through th eopening in the caliper and pry it to push the pistons back. If you do it with the bleeder openthe fluid will be expelled rather than pushed into the lines.
The rear will require you to push each side in as the caliper is fixed. Again a screwdriver against the rotor being careful not to gouge anything.
There are expanders sold for this purpose also.

[xspeedy]

Thanks Ross. Rears are fixed? I recall mine as floating - 94 525i. I'll give the screwdriver trick a shot.

[93 525 Paul]

A lot of the crappy fluid will remain in the bores of the calipers. If you crack open a bleeder now after it had a chance to mingle with the new you'll see. The new fluid doesn't circulate through the caliper but takes the easiest path to the bleeder.

Hmmm. I'll give that a shot. Seems counterintuitive that clean fluid can be in close proximity with dirty and not intermingle.