I have a 2008 Honda Civic LX automatic transmission sedan with 130,000 miles. I noticed the brake pedal was feeling weird, but still drivable. I performed an oil change and noticed fluid under the car (not on the ground) on the drivers side. Looked like fluid had splashed when I looked up from underneath into the underside of the car. I checked under the hood and the brake fluid reservoir looked empty. I checked my front brake pads on both sides and they look fine.

Under the hood, I noticed a fluid/moisture between the valve cover and the air box that holds the air filter. I attempted to remove the air box (I watched YouTube videos) but was unsuccessful. The rubber parts were very difficult to free up, and the connection on the left (the one that isn't a bolt) is tricky as well.

Anyway, if anyone has any ideas of what might be leaking, maybe I don't need to remove the air box. I have attached photos of the moisture I see. I wasn't able to get any shots from under the car. I assume it's brake fluid? Maybe I need a new master cylinder?

I apologize in advance as I am not an expert on car parts and what they're called. Note: I replaced the valve cover gasket in April, 2020, so I assume it's not oil? I bled the brakes with new DOT 3 fluid in April, 2020 as well.

EDIT: I was able to remove the air box! I have attached photos that show the moisture. It appears to be coming from the left of the air box. I assume it's brake fluid but I'm not 100% sure. It looks like the valve cover gasket is still in good shape, and not leaking from there. I checked the master cylinder and it looks dry. There is a hose that runs from that big black circle (brake booster??) to the back of the engine, and that seems dry too.



I have attached photos and hopefully someone recognizes where the leak is coming from.


Thanks!

 

Now that you have the air filter removed can you get a close up on where it is leaking from. It might be the master cylinder seal to the brake booster but this is really uncommon on Hondas but not impossible I suppose.

 

Everything in this photo looked dry....



Are you thinking the moisture in the other photos is not brake fluid?

 

If you unbolt the 4 bolts holding the master cylinder to the brake booster you should be able to see if brake fluid is leaking into the brake booster by gently pulling it back. This assumes you have no leaks you can find elsewhere.

 

I got a tip that the moisture to the left of the airbox is probably oil. I was starting to think the same, because I don't know how brake fluid would be at that location. The camshaft seal is on the other side and it's dry, and the valve cover gasket looks dry, but I learned that there is a camshaft thrust plate that has a seal/o ring that can go bad. I'm thinking that's the issue causing the moisture. Here is a video of the repair:
Hopefully that solves the problem of the leak to the left of the air box, but I still need to figure out why my brake fluid reservoir was empty.

I'll try to undo the 4 bolts you mentioned and see if it's leaking there. I think my car has 2 bolts holding the master cylinder to the brake booster, not 4. I think there might be 4 bolts inside the car behind the brake pedal holding the booster on. Are those the 4 bolts you are referring to? It seems dry where the master cylinder meets the brake booster.

I'm going to try to drive it around my neighborhood to see if the brake fluid leaks, and maybe I can see where it's leaking. That might take some time though with the holidays, and I might want to fix the oil leak first.

 

Yes, there's 2 not 4. There is a seal that can go bad there and it will allow the brake fluid to leak inside the brake booster.

 

Just to follow up. I replaced the camshaft thrust plate seal (as seen in the youtube video) and the moisture on that side of the engine is gone.

A month ago, I went to change my brake pads and rotors and found the drivers side caliper piston dust boot seal was torn and dangling. The small rubber bleeder cap was also torn, and my driver's side cv axle boot had a huge gash in it. So... I think rocks must have sprayed up into the drivers side wheel and ripped three of the rubber parts, causing brake fluid to leak. I have replaced the cv axle and I rebuilt the caliper with a new dust boot seal and bleeder cap. Hopefully they'll work properly and no more brake fluid will leak. I assume the brake fluid loss was from the piston dust boot seal tearing open?? The strange thing is, when I added brake fluid, before I knew the dust boot was torn and dangling, the brake fluid reservoir remained full and the level didn't drop. How did it not pour all over the place when the seal was torn on the piston?