2004 Mustang V6 3.9L 40th Anniversary Edition Brake Fluid Leak


New Member
Apr 23, 2022
Cincinnati, OH
Hello all.

I own a 2004 Mustang V6 3.9L . I recently noticed that my Brake fluid has been leaking a lot. To the point where I'm putting in half a bottle of Dot 3 daily, just to keep the levels where they should be. I tried to find where the leak was, and after looking around the only place where I could see any moisture or brake fluid leaking was coming from this place on the firewall behind the passenger side exhaust manifold. I could only describe it as a rubber and plastic round disc, with a hole in the middle. I saw it leaking from there when I had a buddy press the brake and noticed it continuously come out from there. (The red circle is where I spotted the leak.)

I'm not expert mechanic, I'm just trying to learn and I've changed a few things on the car, like said exhaust manifold, wheels, brakes, etc. However, I haven't messed with the engine besides replacing the ignition system with just newer plugs, wires, and coil. Nothing I messed with should have affected this. The brake fluid was leaking well before I changed all 4 corners with a power stop kit. However, it only just recently has gotten worse. This might not even be the issue, but this is the only thing to me that could be the issue. I've included pictures, any help would be greatly appreciated.


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That's the HVAC drain for moisture that drips out when the AC runs.

If you are losing fluid and cant find the source, check the master cylinder. It's common for the MC seal to leak at the rear, in which the fluid leaks down the underside of the brake booster, or inside the brake booster. You may need to unbolt the MC and slightly pry it forward to inspect, but usually looking directly under it will confirm. Brake fluid will peel paint, and corrode metal (due to absorbing moisture) so look for signs of wrinkled paint/and or rust.