Any idea what has caused this "blue slime" in my car's brake fluid reservoir and brake master cylinder? I purchased the car last August and found the slime when I replaced the rear brake pads, master cylinder and flushed the brake fluid.
Dot3 and dot4 are compatible and mixable. Difference being dot4 has a little higher temp rating.
However dot5 IS NOT compatible in any way with the other 2, do not mix them.
Dot5 is also not hydroscopic and will not absorb water/condensation in the lines like 3/4will.
Doesn’t explain the blue color though I’d guess like mentioned already that it’s washer fluid contamination. Flush the living daylights out of the system and don’t be surprised if you end up needing a new master cyl or other components.
I would remove the MC and bench bleed it, open all the bleeders, then with the open brake lines I would spray brake clean down the open lines or get a large syringe or funnel with a clear hose on it and add brake clean until it came out of the bleeders. Then shoot compressed air from the front open brake lines. Then gravity bleed with your favorite brake fluid, close bleeders, then bleed them one by one.