Silverstone2k4

For several years now I've needed to add brake fluid to the master cylinder reservoir every few months. The level only drops about an inch or so every 3 months. I have been flushing the brakes as recommended every 3 years.

I think I've determined where the missing fluid is going. Around the joint of where the master cylinder meets the brake booster, the black paint shows bubbling. Additionally, there is some rust on the brake booster immediately below the master cylinder. I think the brake fluid is leaking out at this seam.

What is the fix for this? Is it as simple as replacing the rod seal? (#4 on this diagram)

Or will I need to replace the entire master cylinder and/or brake booster?

At this point the issue is mainly cosmetic (corrosion) but I do want to address this before it gets worse. I appreciate the advice.

Gottabfast

That seal shouldn't be seeing fluid pressure, so your master cylinder has a slow leak. If the booster is rusting on the inside from brake fluid, it's only a matter of time before it loses seal and you have to change it out too...sorry for the bad news

Jason99

This is correct. The #4 part you mentioned seals the primary vacuum chamber from the atmosphere. The cup seal or bore inside the master cylinder must be damaged for brake fluid to be leaking out. Your only option is to replace the master cylinder.

Silverstone2k4

Thanks guys. Didn't realize that seal was for vacuum, not to keep brake fluid in.

Time to order a new master cylinder. Any good DIYs on here for replacing it?

Jim21680

I just found a leak in my booster as well (2008 / 95K miles). I only noticed it because of the hissing sound I'd hear for a few seconds after shutting the engine off...I haven't felt any change in brake feel or performance.
I'm still not sure what caused this, but I'll pop the master off this week for a closer look. Finding this leak was driving me absolutely nuts.
I assumed it was cracked hose somewhere, and searching through the abyss of crap in that area, I definitely saw some parts of this engine I've never seen before.

Jim21680

As suspected, it was this little $4 dollar seal causing the vacuum loss. Somehow I didn't see it the first time I looked at the parts diagram, but at least now I have a better understanding of how the booster works.
It's always a good learning experience taking something apart and putting it back together again, right?

vortamock

Interesting. Glad you got it fixed and thanks for sharing!

cosmomiller

Why is there sand all over the top of the booster?

Jim21680

It's just dust and dirt that's built up over the years. My engine bay is mechanically well-cared for, but otherwise filthy.

I'll admit...it is rather shaming for me to google "S2000 engine bay", and see how obnoxiously clean some of these engines are. Maybe I'll pay a little more attention to that going forward...