O fluido de freio está baixo.

In Portuguese, a very common way to form compound nouns is noun + de + noun.

So fluido de freio literally looks like fluid of brake, but in natural English it means brake fluid.

Here, de freio tells you what kind of fluid it is: fluid used in the braking system.

Other similar patterns:

• óleo de motor = motor oil / engine oil
• luz de freio = brake light
• pedal de freio = brake pedal

So de does not always translate as a literal of. Very often it is just how Portuguese links two nouns.

O is the masculine singular definite article, meaning the.

So:

• o fluido = the fluid

Portuguese uses articles more often than English does. In a normal full sentence, O fluido de freio está baixo sounds natural.

In short warnings, labels, or dashboard messages, the article may be dropped:

• Fluido de freio baixo

But in regular speech or standard sentence form, O is expected.

Portuguese has two main verbs for to be: ser and estar.

Here you use estar because the idea is a current condition or state:

• the brake fluid level is low right now

So:

• está baixo = is low as a present condition

You would not normally use é baixo here, because ser suggests a more inherent or permanent characteristic. That does not fit a fluid level warning.

A useful rule of thumb:

• ser = identity, definition, more permanent traits
• estar = condition, state, situation

Because baixo agrees with o fluido, which is masculine singular.

Agreement in Portuguese:

• masculine singular: baixo
• feminine singular: baixa
• masculine plural: baixos
• feminine plural: baixas

So:

• o fluido está baixo
• a pressão está baixa

Even though in English you may think of the level being low, the sentence as written has fluido as the grammatical subject, so the adjective matches fluido.

Not literally. Here baixo means low in level, amount, or reading.

So está baixo is the natural way to say that something like a level, amount, pressure, battery charge, or reading is low.

Examples:

• A pressão está baixa. = The pressure is low.
• O nível da água está baixo. = The water level is low.
• O fluido de freio está baixo. = The brake fluid is low.

So yes, baixo can refer to a measured level, not just physical position.

Yes, that is also very natural, and in some contexts it is even clearer.

Compare:

• O fluido de freio está baixo.
• O nível do fluido de freio está baixo.

The second version is more explicit, because what is really low is the level of the fluid.

Still, the shorter sentence is common and easy to understand, especially in automotive contexts. Portuguese often allows this kind of shorthand, just like English does with brake fluid is low.

Because freio here usually refers to the braking system in a general sense, not to one individual brake part.

This is very common in Portuguese. The singular can refer to the system or function:

• fluido de freio = brake fluid
• luz de freio = brake light
• pedal de freio = brake pedal

So singular freio is the normal form in this expression.

In normal speech, that sounds less natural than O fluido de freio está baixo.

However, in certain special contexts, dropping the article is common:

• dashboard alerts
• repair notes
• labels
• headlines
• checklist-style language

So:

• full sentence: O fluido de freio está baixo.
• warning/alert style: Fluido de freio baixo.

Both can appear, but the version with O is the better default for learners.

Usually, baixo is better here.

Why? Because Portuguese often uses baixo for levels, readings, and measured amounts:

• nível baixo
• pressão baixa
• temperatura baixa
• combustível baixo in some warning-style language

Pouco means little / not much, but it does not fit as naturally in this structure:

• O fluido de freio está baixo. = natural
• Há pouco fluido de freio. = possible, but a different structure
• Tem pouco fluido de freio. = common in informal speech

So if you want to talk about a low level, baixo is the normal choice.

Yes, fluido de freio is a standard and very normal term.

You may also hear óleo de freio in everyday speech, although technically that is less precise, since brake fluid is not literally an oil in the usual sense.

For learners, fluido de freio is a very good standard expression to know.

A rough pronunciation guide is:

oo FLOO-ee-doo djee FRAY-oh es-TAH BAI-shoo

A few notes:

• O sounds like oo
• fluido is roughly FLOO-ee-doo
• de before f is usually like djee or a softer jee/dee depending on accent, but many speakers pronounce it more simply as de
• freio is roughly FRAY-oh
• está stresses the second syllable: es-TAH
• baixo is roughly BAI-shoo

Pronunciation varies by region, but that approximation will be understood well in Brazil.