When I took a look under the hood of a friend's car, I noticed that the level of brake fluid in master cylinder was slightly above the MAX line. Not by that much, but clearly noticeable. I'll probably remove for her just because MAX means MAX, but in meantime, since it's been driven that way for a while, are there any actual impacts or issues to be expected or that she should look out for?
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No issues or impacts if it's slightly overfilled, which I'll define as 1/4 inch or 0.5 cm.
The reason there needs to be a little headroom is that brake fluid expands when it heats up, as all fluids do. If it were grossly overfilled and your friend were overusing her brakes, for example in mountain roads, rather than using engine braking, there would be a chance that the brake fluid would overflow the reservoir. This could lead to softened paint under the hood and future rust. I suppose there could be a fire risk if the overflow dripped onto the exhaust system, but this is extremely unlikely except for a race car.
Me personally, I would leave it alone unless it's grossly overfilled.
MTA
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The worst thing which can happen if the brake master is overfilled is when the brakes are done, it will overflow as the pistons of the calipers are retracted back into the body of the caliper. No physical damage will happen to the master or brakes due to the master being overfilled. There's no safety issue there either. It will not degrade the performance of the brakes, either.
Pᴀᴜʟsᴛᴇʀ2
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It may even not be overfilled in the first place.
The level changes with brake pad change and temperature.
If it was topped up in a moment when the level is naturally low (worn brake pads, cold weather) it will get somewhat above the MAX when it is hot and/or when new pads are installed.
The MAX mark is to be considered only when you top it up.
A very high level or overflowing could be a sign of a brake system malfunction like air in the system or overheated brakes in this very moment, but both conditions will manifest in degraded brake performance first.
The paint considerations in the other answers are imho outdated - modern (after 1990) car body coatings are generally resistant to DOT 4/5/5.1 brake fluids. Guess how I found out. And even if the paint right below the master cylinder suffers somehow, rust in this particular area is highly unlikely.
fraxinus
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Overfilling the master cylinder can cause brake fluid to overflow which then leads to more serious issues.. causing leaks and serious damage like brake failure. As fluid heats up, it expands and causes excess pressure leading to inconsistent braking. Don’t ask me why I know this. Also remember to clean up any spilled brake fluid because it is corrosive and can damage paint/components.
Muzzammil Riaz
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