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Brake master cylinder cap

2K views 4 replies 3 participants last post by  dp0099  
#1 ·
Hello,

Last week I did brake fluid flush on our 2022 Pilot. This was the first time I opened the master cylinder cap, in the less of 3 years of owning the car.
The brake fluid was very dark.
Under the strainer, the lip of the reservoir, was very dirty - looked as if dirt was sucked in from the outside.
The cap rubber seal was not sitting flush on the cap, but one side was pulled away from the cap.
I flushed the system and cleaned the dirt.

I am not sure what the issue is here - defective cap seal or/and defective cap or/and reservoir or/and poor assembly at the factory.
In any case, I decided to replace the cap seal (with aftermarket, direct replacement for the exact Honda part). When I went to replace it, the OEM seal, again was not flush with the cap. Will see how the aftermarket one will work.

Tried to raise the issue with dealer ... got some nonsense that it is normal to have dirt like that, and if there is a leak from the cap I will be getting warning lights.

Has anyone seen something like this on a less than 3 years old car?
Is there a way to quickly check if the cap itself maybe bad?
The top edge of the master cylinder, where the rubber seal contacts the cylinder plastic body, is all smooth. No irregularities.
 
#2 ·
If you are talking about the rubber insert which is on the underside of the reservoir cap, that is normal for it to be pulled down. It gets pulled down as the brake fluid is drawn through the reservoir by normal braking. I also did my brake fluid flush in the last month or so and was surprised at how dirty the strainer was.

I had done the first flush just about 3 years earlier and it was also dirty at that time. I don't believe dirt is getting in, I just think it is normal for the fluid to leave a dirty looking residual. My car has 57000 miles, is a 2019 model, and this was the second brake flush I did and everything is operating as it should. I just push the rubber lip back into place and then put the cap back on. I have always used the Honda brake fluid.
 
#3 ·
Yes, that one.
With the other cars I maintain, even after 4 years of no brake fluid flush, with significantly more driving, brake fluid has never been dark at all. And no dirt whatsoever.

In my case, the strainer was clean. But the lip the strainer sits on - this is where the dirt accumulation was - the entire circumference.
The fluid itself did not contain any particles/debris I could see. Only very dark.
I use only Honda fluids (except for the engine oil). My 22 Pilot has less than 15000 miles.
 
#4 ·
Brake fluid is hydroscopic so it absorbs water and heat cycling + age turns it darker.

yes normal for the cap to do that, brake pads wear so the brake fluid level goes down, the cap "expands" to compensate for the master cylinder to keep the same air gap. When the pad wears and fluid goes down it pulls a small vacuum down from the cap that's the expansion since it's a sealed system. Since it pulls a vacuum with brake wear, it could pull dirt around the cap threads... - that's what I'm thinking since there's a strainer in the Honda's I worked on.
 
#5 ·
The cap seal that came with the car has a tiny cut in the middle. I think that would allow air (moisture and dirt) to be sucked into the reservoir as the brake fluid level changes.
My understanding is that the cap seal must be flexible to allow for the break fluid expansion/contraction, while at the same time does not allow air to enter the reservoir.
I do not see how, having a tiny opening in the seal, will not allow air (moisture) from entering.
The aftermarket rubber seal does not have cut in the middle.