NachoLibre

After how many miles I need to change the brake fluid on 2011 e550? The dealer told me every 20k. I got a lexus gs with 160k and still original brake fluid, but the ABS unit did go out once and was costly repair.

should it be ok if I flush it every 50k or so.

Mud

40-50k is about the norm. I just did mine at 50k miles, maybe 30-40 if you drive the car more aggressively. The fluid in the reservoir can look good but its the fluid in the calipers and lower brake lines that really sees the heat cycles and debris from rubber parts. I did mine as part of an overall brake refurbish, seems that brake work timing coincides with being about right to change fluid as well.

MnRiverman

Every two years for brake fluid flush is the norm due to possible moisture contamination in the system.

Mud

I think most mfg cover the bases with the either-or scenario. Miles or time. For me I go by miles as I've seen more issues from debris than moisture or whether its been a couple of years since last flush. But you're right that timeline is also a factor and should be mentioned.

cetialpha5

For MB, it's every two years which is why a brake fluid flush is part of Service B. Brake fluid will naturally absorb water through seals, hoses etc., can't be permanently stop. Dot 3 isn't as hydroscopic as Dot 4 or Dot 4+ which is what MB uses so you were able to get away with not flushing the lines over a longer period and some manufacturers never recommended an interval in the old days. Even if you don't drive the car, the brake fluid is still absorbing moisture from the atmosphere, that's why you see two ratings on the brake fluid, dry and wet boiling. For instance Pentosin super dot 4 has a dry boiling point of 509F, but the wet boiling point is 329F. Eventually you may absorb enough moisture where it's just water and once the brake fluid boils, you don't have any brakes because you can compress a gas, but not a liquid.

Mud

Yes I understand and don't disagree. The difference in the dry/ wet boiling points shows how much degradation can happen. I need to look up what % water can lower the boiling to that level.

kajtek1

Living in dry climates I got away with 5 years intervals, but each time I replace brake pads, I push the fluid from calipers outside and top off with fresh one.
As stated- fluid absorbs water and it is time-related process where mileage does not matter much.